Purpose: This study addresses a gap in the knowledge on how longitudinal engagement in a school improvement initiative influences change in middle leaders' (MLs') interactions and assesses how school-university partnerships around school improvement can support teachers with formal leadership roles (i.e. MLs') leadership development. Design/methodology/approach: Using a two-year longitudinal research design, university staff facilitated middle leadership training in a school-defined improvement initiative on lesson study. Results from a pre-test followed by two post-tests administered at one-year intervals were collected on social networks. Analyses examined changes in indegree and brokerage patterns among groupings of senior leaders (SL), subject leaders, cross-school specialists and teachers. Findings: Accounting for staffing changes, 27 of 67 staff members participated in each survey, yielding 1,623 distinct ties connecting school members. Over the first year, advice-seeking increased by 225%. SLs' initial propensity to consult peers shifted towards MLs and teachers. Subject leaders advising other leaders and teachers increased tenfold. Teachers' peer-to-peer consultation increased by 2,000%. Specialists with school-wide responsibilities became the dominant group for advising other leaders, such as SLs and subject leaders. These shifts were sustained over the second year. Originality/value: The study demonstrates that engagement in the school-university partnership support and the corresponding structural changes stimulated robust cross-school dialogue among teachers and various leaders. Brokerage patterns indicated an enhanced role for MLs in driving the school-defined improvement initiative which corresponded to university-designed development activities.