At Hecate Hall, a.k.a. Juvie for Monsters, it meant I was getting thrown in the pond.
Hall = High School?
What does Juvie mean? What can I say for Juvie?
Hmm. Hall does not literally mean "high school". A "Hall" (with capital H) is simply a large building of some kind, often a manor house or stately home.
Combined with another word like "Hecate Hall" then it sounds like the name of a building. Hecate is a goddess of witchcraft, in case you didn't know.
You couldn't use "Hall" instead of High School in a sentence.
Given the limited context I would guess that you must be reading some form of Harry Potter style fiction? In which case "Hecate Hall" is probably the name of the school, and since it is "for monsters" it's presumably a school which is attended by young wizards/witches/werewolves/vampires and the like. Such supernatural creatures are often referred to as monsters.
"Juvie" is an abbreviation for "Juvenile hall" which is an American term for a children's prison or detention centre. Therefore the word is used to imply that the school Hecate Hall is like a prison and not a nice place to be.
This is supported by the fact that if the speaker was at a "normal" high school, having a lesson outside would be fun. Having a lesson outside at Hecate Hall is not a fun experience, as s/he has ended up being thrown into a pond.