All of those prepositions work, but not all of them convey the same meaning.
Teachers are in favor of banning plastic from Maximilian Gymnasium.
This is the most natural preposition and likely captures your intended meaning: that teachers want the school to enforce a "no plastic" policy. "Banning/banned from" is a fairly standard usage. For example:
I've been banned from the chat room.
Teachers are in favor of banning plastic at Maximilian Gymnasium.
Essentially conveys the same meaning as 1. You could maybe argue that the meaning is more along the lines of, "We will not tolerate the use of plastic at our school." as opposed to "Eliminate all current and future plastic from these grounds." But I think that's a stretch, and it's more or less identical to 1.
Edit: note, however, that its usage depends on the context. It would not be natural, for example, to say "I've been banned at the chat room."
Teachers are in favor of banning plastic in Maximilian Gymnasium.
Though grammatically correct, this is likely not the intended meaning in your example. The preposition "in" should only be used in place of "at" when it's literally referring to being inside a given place or institution (i.e., within the walls of the school building).
Here, the meaning is definitely different from those of the others: in particular, it could imply that the use of plastic outside the school, but still on school grounds, would be acceptable.