that's a discussion you need to drag your parents and the principal into...teachers are supposed to use proper judgement...and there are students who use the restroom as an excuse the proceed to get into trouble which gets the teacher in hot water...one argument against you is that there is ample time to take care of such things between classes...although there is no controlling when a bodily function is going to kick in...i know that as a general rule of thumb, students who are not disruptive in class and tend to be attentive getting decent grades, unless the teacher is a total donkey the good students tend to get more leniency in such things...
if the time between classes is not enough...try getting to class a bit early then inform the teacher of your need and that you will return a quickly as possible...even for the turd students, there is less trouble.
it is not a human rights issue...