Maybe she asked whose quarter sheet was lying out and someone told her it had been there for days? She's the BM, it's her job to keep the place clean and organised, and the snide remarks might just have been annoyance at having to clean up after you. It seems a bit paranoid to assume it must have been deliberately hidden so that you couldn't find it. Maybe she just put it away somewhere you didn't think to look.

Anyway, you're not communicating with her either, so if it's high school then the two of you are in the same class.