Go Banana = becomes crazy; Go Toilet ???

"to" gives readers a sense of action done towards a place. Here, "go" is an intransitive verb (i.e. no object; nothing to accept/receive the action). One can make a toilet, break a toilet, flush the toilet (all transitive verbs) but 'go a toilet' doesn't make any sense. So one has to say "I go to school by bus."

Another meaning of "go" is "to be". For example: "go bananas" means "go crazy", i.e. to become crazy. I don't think your teachers would want to know if you want to be a toilet.

Sometimes, we go shopping, we go swimming ... Here, shopping and swimming are activities. Toilet is a thing. So you should say "Teacher, may I go wee wee/poo poo?" which is like in Chinese 先生, 我想痾屎!  

http://dictionary.cambridge.org/dictionary/british/go_1
http://dictionary.cambridge.org/dictionary/british/to_2
http://dictionary.cambridge.org/dictionary/british/to_3