Urban areas across Europe are characterized by increased diversity. This is reflected in the school populations in these areas with a high degree of heterogeneity and diverse learning needs (Smets & Struyven, 2020). Secondary school teachers are expected to adapt their teaching to the diverse educational needs of students through differentiated instruction (DI) (OECD, 2018; UNESCO, 2017). DI is seen as an important domain of educational quality and as an approach for providing equal educational opportunities for all learners (Brevik et al., 2017). According to Tomlinson and Imbeau (2010), the core of DI is the adaptation of learning content, process, product and effect in response to differences in learners' readiness, interests and learning profile. This involves teachers assuming different learning needs in their 1) lesson planning and preparation, 2) selecting materials, 3) lesson activities, 4) classroom organization, and 5) student evaluation and assessment (Prast et al., 2015). DI is included in teachers' competence requirements and part of the educational inspection framework. Most teachers recognize the different learning needs in the classroom and the need to adapt instruction, but few secondary school teachers actually put DI into practice (Inspectie van het Onderwijs, 2017). Letzel et al. (2023) found that teachers in upper secondary schools are more often than not low on the implementation of DI, and do not often implement DI. Several explanations are offered in the literature: Teachers indicate that they do not feel prepared to implement DI, that they lack knowledge and understanding of DI. They find differentiation too complex to incorporate into their teaching practice and doubt its feasibility (Gaitas & Alvas Martins, 2017; Whipple, 2012). Research shows that teacher professional development for teachers is warranted. Langelaan et al. (2024) identified characteristics of teacher programs for DI that were successful to some extent. They incorporated active learning, collaboration and reflection and were often longitudinal, comprehensive and addressed attitudes, knowledge and skills. Understanding of differentiation is cited as a prerequisite for being able to apply DI in practice and teachers' attitudes play a crucial role in their use of differentiated instructional practices (Wan, 2016). Educational innovation and improvement is generally seen as intricate: professional learning of teachers within the school is constantly influenced by the often complex school context in which it is implemented. Factors like the (national) curriculum, policies, various organizational factors, and numerous personal characteristics of teachers and students constantly interplay with school development processes. Dack (2019) found that knowledge about differentiation of candidates developed during a course and that participants integrated the newly acquired knowledge into their existing frameworks of knowledge about teaching and learning, which strengthened their beliefs about the importance of differentiation and their learning.  At the same time, teachers are constantly trying to find coherence between their own personal frames of reference and those of the changing context during an innovation (Stollman et al. , 2022). As a result, educational innovations often turn out differently than they were intended by their developers. TPD programs for DI are often initiated and developed by external parties and facilitated by educational experts from outside the school (Dixon et al., 2014). Educational development and improvement driven by the school's own ambition may lead to more sustainable and lasting change (Geijsel et al., 2009). At the same time, the question arises whether schools have the resources, capacity and expertise needed . research question: how and to what extent does a school-led and  school driven innovation for DI contribute to teachers conceptions, attitudes and classroom practices concerning differentiated instruction?  The innovation was monitored during the course of one school year.

The initiation of the innovation was an internally driven endeavor undertaken by the school itself, adopting a participatory approach informed by theoretical insights of successful DI implementation. This study employed a longitudinal mixed methods research design within an explanatory sequential framework to thoroughly examine the contribution of the intervention. Specifically, we focused on three key dimensions: (1) teachers' conceptions (2) the extent of DI implementation in the classroom, and (3) the attitudinal aspects of teachers toward DI. The data collection instrument utilized was a comprehensive self-completed questionnaire comprising 62 items, including one open-ended question and 61 closed-ended items measuring 10 variables. The data were collected from a cohort of 62 teachers at three distinct measurement points throughout a school year. To assess the impact of the intervention, the results from these three measurements were subjected to statistical analysis using a repeated measures ANOVA. Post-intervention group interviews were conducted with a representative sample of 21 teachers to delve deeper into the contribution of the intervention and to identify hindering and supporting factors. A content analysis approach was applied to scrutinize the qualitative data obtained from open-ended questions probing teachers' conceptions of DI. In the subsequent qualitative research phase, the focus shifted towards a nuanced understanding of the learning process and implementation of DI. This phase employed instruments and analytical methods tailored to explore social processes and meaning-making. Thematic analysis was employed iteratively in line with the principles of qualitative study research to extract meaningful insights from the data. This methodological approach allowed for a more holistic understanding of the complex dynamics involved in the adoption and integration of DI practices within the educational context. By combining both quantitative and qualitative research paradigms, we aimed to paint a comprehensive picture and articulate statements regarding the multifaceted factors that either facilitated or hindered the teachers' learning journey.

The findings elucidate the impact of innovative measures on DI with regard to conceptualizations, attitudes, instructional practices among educators, and the factors influencing both the learning of DI by teachers and its subsequent implementation. 1) Over the course of the academic year, there was a discernible progression in teachers' DI conceptualizations, exhibiting greater comprehensiveness and alignment both internally and with established literature definitions. Utilizing a repeated measures Analysis of Variance (ANOVA) with consideration for sphericity, it was observed that teachers' engagement in differentiation planning, preparation, utilization of activities and materials, as well as their mindset exhibited statistically significant growth on average across distinct time points. Conversely, no statistically significant differences were detected between the various measurement times in relation to the variables of classroom environment, organization and management and assessment and evaluation.. In the subsequent qualitative research phase of data collection, the emphasis was placed on gaining a deeper understanding of the learning process and implementation of DI. The study employed specialized instruments and analytical methods centered on social processes and meaning-making. This qualitative approach aimed to explore the nuanced aspects of how educators engage with and interpret DI, shedding light on the intricate social dynamics and sense-making processes inherent in the implementation of innovative pedagogical practice.

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