In the Northeast part of the United States (and quite frankly this is more common in the US period), we say, "Can I go to the bathroom/restroom?" or "I need to go to the bathroom/restroom." Perhaps saying "Excuse me" or the person's name (ex. Mrs. Smith) first to get his or her attention. We're pretty informal about things so most people don't normally say "Miss/Sir/Ma'am". In my area we don't call the bathroom the "loo" or "toilet".

Where I am, you'd probably just say "Miss/Sir can I go to the toilet" or "I need the toilet". If teachers refuse girls just use the old "I'm on my period" excuse (not going to lie I've been known to do that a couple of times) :D

In the US, we rarely use the word "toilet" for this. Usually, we would say, "Excuse me, can I go to the ladies' room" (or the mens' room) or "Can I go to the restroom?" If it is a classroom of adult students, they usually just say "May I be excused?" or "Please excuse me" or "I'll be right back." Americans usually avoid the word "toilet" and sometimes "bathroom" when they are trying to be polite.

Teacher i need to take a leak for a while. You may be so mean.

Mr. Or Mrs., may I use a restroom real quick?