Seriously, take the frog. Leave the tank.

What is the teacher going to say??? "I left the class pet frog here for the summer with no one to feed it, and now it's gone!" Not gonna happen.

As a teacher myself, I do know that many other teachers make arrangements for the pets to go home with a student(s) over the summer if they are not their own pets. I know the end of the year, and the last day especially, is a busy one, but it's NOT that busy that you forget a pet. Shame on them.