The Pennsylvania Department of Health notified the Norwin School District that one student has the highly contagious virus, prompting school officials to pull student immunization records.
Thirty-three students were found to not have the vaccine, though at least one had been vaccinated later Thursday, after the school district sent a letter home to parents and posted the health department information on its website.
Norwin's prom is Friday night.
School officials say students without the vaccine also can't attend classes through the incubation period that runs through May 28. The can return to school before then if a blood test proves they're immune or if they get the vaccine by Sunday.
