Background: Schools are high-risk settings for influenza transmission. During the 2025-26 Northern Hemisphere season, influenza A(H3N2) subclade K (J.2.4.1) rapidly emerged and became predominant, but vaccine effectiveness (VE) in school-associated outbreaks remains unclear.

Methods: We conducted a field investigation of 93 school-associated febrile outbreaks in Beijing, China, from September to December 2025. Cases were febrile students with laboratory-confirmed influenza, and afebrile classmates served as controls. VE against laboratory-confirmed symptomatic infection was estimated using mixed-effects logistic regression, adjusting for age, sex, school type, and outbreak-level clustering.

Results: A total of 3,797 students were included, of whom 788 were laboratory-confirmed febrile cases and 3,009 were afebrile classmates. Primary schools accounted for 74.2% of outbreaks. Overall vaccination rate was 54.6%, with 50.6% among cases and 56.1% among controls. Unadjusted VE was 19.6% (95% CI: 5.9-31.3). After adjustment, VE against laboratory-confirmed influenza was 30.0% (95% CI: 17.3-40.7). Sensitivity analyses yielded consistent results.

Conclusions: Influenza vaccination provided modest protection against symptomatic A(H3N2) infection in school-aged children, even under conditions of high-intensity exposure and antigenic drift. These findings underscore the value of vaccination in outbreak-prone school settings and highlight the importance of ongoing vaccine strain optimization and outbreak-based VE surveillance.