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

Sleep vs. Nap; Name vs. Lamb

Are You Finished?

I tend to ask students if they 'are finished'. But they laugh as if I'm asking if they're dead.

http://www.bbc.co.uk/worldservice/learningenglish/language/askaboutenglish/2009/06/091006_aae_finished.shtml

Alternatives:

Are you done?

Are you through?

環保－－可能嗎？

最唔環保就係「人」。想環保只有優化人嘅素質，勿過份生育。但系因為人權嘅問題，咁系冇可能。雖然中國嘅 one child policy 產生咗好多問題，但如果當初無推行嘅話，今日會有更多及更嚴重嘅問題。當然從教育入手亦可以，但系要點樣餒當咁教育到每一個人亦系好大嘅問題。教育咗亦未 必從此就冇人唔貪方便、唔自私。10 個環保人都唔知保唔保救到 1 個唔環保嘅人嘅破壞力。地球有セ嘢未試過？冰河時期、大爆炸...... 救地球？救救你自已先啦！好好做個環保嘅家長啦。

To or not to. That's the questions.

Student's sentence: "We should help our parents to clean the house." Her teacher crossed her 'to'. "To or not to. That's the question."

http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv226.shtml