Ample research has consistently highlighted the positive influence of both principal instructional leadership and teacher collaboration on pupil performance. Given the collaborative nature of schools, we sought to investigate whether there is a mediation effect of principal instructional leadership through teacher collaboration on pupil performance. Surprisingly, the results unveiled a counterintuitive pattern—negative relationships emerged. In other words, the combination of effective principal leadership and enhanced teacher collaboration was associated with a negative impact on pupil performance. While this finding may raise eyebrows, it is not an isolated occurrence in the literature, prompting a re-evaluation of the conventional understanding of leadership dynamics in schools, emphasizing the pivotal roles played by both teachers and principals. Our journey into this novel perspective begins with a robust longitudinal dataset encompassing 79,683 teacher evaluations, offering a nuanced exploration of principal leadership and teacher collaboration in shaping the educational landscape.

The significance of principal instructional leadership in education has been extensively discussed, particularly its impact on pupil academic achievement (e.g., Böhlmark et al., 2016). Effective instructional leadership sets high academic standards and nurtures a conducive learning environment, vital for student success. Principals who prioritise academic goals, provide teacher support, and foster a collaborative culture can significantly enhance school performance. However, empirical research is lacking, particularly concerning the interplay between instructional leadership, teacher collaboration, and pupil outcomes.

In educational research, instructional leadership is a cornerstone concept, focusing on principals' roles in shaping teaching and learning environments. This involves defining educational goals, developing pedagogical programs, and fostering a conducive learning climate. In Sweden, where this study is based, school leadership research has been limited despite the acknowledged importance of principal leadership for school performance (Ärlestig et al., 2016).

Principal leadership is believed to indirectly affect pupil outcomes through various channels, including teacher morale and school culture. Meta-analyses have consistently demonstrated positive associations between principal leadership and pupil achievement (Wu & Shen, 2020; Tan et al., 2020). However, the exact mechanisms through which leadership influences academic outcomes remain complex, with potential moderating factors such as school context.

Teacher collaboration is critical to effective school environments, facilitating resource exchange, targeted student learning discussions, and mutual learning experiences (e.g., Banerjee et al., 2017; Vangrieken et al., 2015). Principals play a vital role in fostering a culture of collaboration (Voelkel, 2022), which has been shown to impact pupil achievement positively (Goddard et al., 2007). However, recent research has yielded mixed results regarding the relationship between teacher collaboration and pupil performance (Goddard et al., 2010; Mora-Ruano et al., 2021), necessitating further exploration.

This study investigates the relationship between principal instructional leadership, teacher collaboration, and pupil academic achievement. This research seeks to elucidate the intricate dynamics of school leadership and its impact on student achievement by assessing principal leadership through teacher ratings and examining its association with pupil outcomes. Additionally, the study aims to explore the potential indirect effects of instructional leadership on pupil performance through teacher collaboration while controlling for various confounding variables.

In sum, this research provides valuable insights into the role of principal instructional leadership in shaping school environments and influencing pupil achievement. By elucidating the mechanisms through which leadership impacts student outcomes, this study aims to inform educational policy and practice, ultimately enhancing school performance and student success.

The data for the study was collected from a teacher survey conducted by the Swedish School Inspectorate, distributed biannually to all schools. From an initial pool of n=243,880 responses, n=79,683 were included, as only schools serving the final year of compulsory schooling were included. The sample was nested within n=1,643 schools, generating n=3486 school scores for Principal instructional leadership and Teacher collaboration. Collected longitudinally over five years (2015-2019), the data averaged n=2.1 responses per school. Responses for Principal instructional leadership and Teacher collaboration were obtained simultaneously, with each school's data gathered on multiple occasions. While school data often exhibit a multilevel structure in cross-sectional studies due to within-school variability, this dataset solely provided school-scale scores because individual teachers were not tracked over time. However, the longitudinal nature of the data still permits multilevel analysis due to within-school variability over time. No missing data were present, as only complete survey responses were recorded. The independent variable, Principal instructional leadership, was measured with three items showing good internal consistency (α=.88) and an intraclass correlation coefficient (ICC) of .27 on the first occasion (2015). The mediator, Teacher collaboration, comprised three items with moderate internal consistency (α=.61) and ICC of .20 on the same occasion. School averages per occasion were used for both outcomes of interest, i.e., Grades and Standardised test scores. Grades were furthermore averages of the three subjects of Swedish, Math and English. We used multilevel structural equation models (MSEM) (Preacher et al., 2010) to examine the effects of Principal instructional leadership on pupil achievement mediated by Teacher collaboration. In these models, Grades and Test Scores were dependent variables, while SES, Share of certified teachers, and Teacher-to-student ratio were used as observed control variables. Principal instructional leadership and Teacher collaboration were treated as latent variables. The MSEM framework allows for simultaneous, one-step estimation of multivariate models, making it suitable for mediation analyses. For the dependent variables, Grades and Test Scores, unconditional models incorporating time as a covariate were estimated to assess any temporal effects, considering the linear change over the study period. The resulting changes were positive but small.

The findings from our current study reveal consistent trends in grades and standardised test scores as outcome variables. Intriguingly, there is no observed mediation effect of teacher collaboration within schools over time regarding the relationship between principal instructional leadership and school outcomes. However, a noteworthy negative mediating relationship emerges between schools. When control variables are introduced, only the negative relationship between schools in the context of grades as the outcome variable remains significant. This unexpected outcome diverges from prior research expectations regarding the association between principal instructional leadership, teacher collaboration, and pupil performance. Notably, a previous study utilising PISA data (Mora-Ruano et al., 2021) also discovered a negative relationship. To delve deeper into this counterintuitive mediating relationship, we plan to conduct a latent class analysis (LCA) of the data. The objective is to discern whether the unexpected association between principal instructional leadership and teacher collaboration represents a linear trend in the sample or if various trends exist, forming distinct groups of schools with diverse relationships between these variables. This analysis seeks to uncover potential mechanisms that may differ between schools, such as variations in teacher collaboration constellations. While these intricacies may not be explicitly modelled in our data, we aspire to offer valuable insights into potential school differences, urging future research to move beyond replicating linear relationships across entire samples of schools. In summary, while principal instructional leadership and teacher collaboration each exhibit connections with pupil performance, their combined influence, inevitable within the educational setting, introduces nuances. This prompts a call for further investigation in research on principal leadership and teacher collaboration.

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