I've had a cousin that was sent away to a "program for bad kids" (what my parents called it). He was there for a few weeks. When my brother acted up, my dad threatened to send us to military school (my mom said to keep my uncle out of trouble, her parents put him in a private school that had an Army ROTC program) In the 60's, where might you send (or threaten to send) an 11 year old girl for misbehaving and doing bad in school?
Sending misbehaving kids to military school is only an option if you have the money. A more realistic choice, and I might add, more humane and sensible, is to send a child who's troubled or perhaps fallen in with the wrong friends to stay with a relative. I think it would also make for a more interesting story having the kid sent off to an eccentric aunt or uncle.