Lancaster police say Cherlie LaFleur gave birth in the bathroom at McCaskey East High School Tuesday and discarded the male fetus in a trash can after trying to flush it down the toilet.
Investigators don't know if it was stillborn or born alive; an autopsy showed it was 27 to 29 weeks.
LaFfleur is charged with concealing the death of a child. It wasn't immediately clear if she had an attorney.
A custodian found the fetus around 10 p.m. Tuesday. On Wednesday, detectives conducted interviews and reviewed video footage, identifying LaFleur as the mother.