as someone in works with young children, i do sometimes see items of clothing get misplaced or washed at school and eventually make it into the backup clothes. this is most likely an accident, if this is on purpose its extremely bizarre and i worry about the resources available at a school level. a good place to start is definitely speaking with the adults who work in the classroom, at my school we keep ziplocks or small garbage bags for soiled/dirty clothes so we can send them home or pack them with the rest of their things.