What could the principal do?
The teacher doesn't need to be labeled as stupid or ignorant. Everyone starts in the hobby somewhere. I myself started with a betta in a bowl. If it were me, I would not report the teacher to the principal, who probably has the same lack of knowledge as the teacher. It's likely not that the teacher is being ignorant, they were probably just wanting another class pet. You can't blame someone for seeing goldfish in bowls and bettas in cups being promoted all across the pet store and internet and expect them to know better. If it were me, I would type up a short, simple, to the point care sheet covering the basic problems of tank size, filtration, diet, and heater and let the teacher know it is what the fish needs to thrive.
As someone who has personally gone out, bought a bowl, a couple of female bettas, a bag of marbles, and plastic plants, I can tell you it isn't that they don't want to give the betta a good life. I did research before I put my two female bettas in the same bowl, I did research before I bought them! I wanted so badly to do my best for them. I did research, and not being experienced in the fishy world, I of course didn't know what to be looking for. Lots of things I read simply told me female bettas could live together, but didn't say a word about tank size! Newcomers to the hobby simply don't know what to look for. They go to the only place they know about (their local petco or petsmart) see those tiny bowls being promoted for bettas, see those pretty bettas, and there is nothing telling them there is anything wrong with that! I have an aunt who loves her pets and wants whats best for them, and she has a pleco in a 3 gallon because she doesn't know any better! My point is, it doesn't always come from a heart of ignorance. Sometimes the relationship is more important than the fish, sadly. You can do your best, but it isn't worth hurting the relationship.
