Background: Vaccination programs in school health centers (SHCs) may improve adolescent vaccine coverage. We conducted a process evaluation of an intervention to increase SHC-located vaccination to better understand the feasibility and challenges of such interventions. Method: Four SHCs participated in an intervention to increase provision of recommended vaccines to 2,975 adolescents. We reviewed program materials and SHC staff reports, and interviewed parents to assess implementation fidelity and reactions to materials. Results: Ten percent of parents returned forms with consent to at least one vaccine. Of these, 79% checked the box consenting for "all recommended" vaccines, rather than indicating individual vaccines. SHCs sent supplemental mailings to some parents that clarified (a) vaccination costs or (b) human papillomavirus vaccine recommendation for boys and required parents to reconsent. This process resulted in loss of initial consent, primarily due to nonresponse. In interviews, parents who consented to vaccination indicated that intervention materials were clear and persuasive, but needed greater detail about costs and clinic processes. Conclusions: With limited additional investment, it appears feasible for SHCs to achieve a modest increase in the number of vaccinated adolescents. Providing a checkbox to indicate global consent for all recommended vaccinations, and close collaboration among individuals involved in intervention development, may facilitate vaccination efforts.