Hi there,
You should probably ask the permission of your teacher first. Even though it is not the proper way to keep a fish, it's still her fish and therefore you need to ask for her permission to get it a bigger tank. I know some people who get touchy whenever someone changges something for their fish.
As for a tank, a 2.5 gallon will do. Yes, a 5 gallon is bigger and better, but all you need is anything above 2.5 gallons really with proper filtration and daily/weekly mainteance.