one of my classmates went missing in high school

[AidaGlass]

Does the part in red mean that her classmate went to school and never got back home or he went missing when she was a high school student?

Certainly, we didn’t understand what bad things lurked out there. Until one of my classmates went missing in high school. Then another girl from the town over. And four more girls quickly after that. The police caught the killer when I was twenty-five.

Before She Disappeared by Lisa Gardner

 

[Egmont]

It probably means that it happened during the three or four* years that the writer was a high school student. The phrase is ambiguous and could mean that the person specifically went missing during the hours that he or she was in school, but I don't think that's as likely.
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*In the U.S. it could be either, depending on where and when this story is set.