Students are encouraged to use the restroom and to get a drink of water at times when direct instruction or testing is not taking place. They do no need to ask for permission. Sometimes, an "emergency" may occur, and I have reassured students that if that is the case, they may go directly to the restroom as needed, no matter what is going on at that time.
Recess is right before lunch and students are encouraged to get water and to use the restroom, as long as they let the teacher know that they are going and that they travel with a partner. Upon return, I've asked my students to simply let me know that they have returned.
We have bathroom procedures to allow for students to "do what they need to do" and at the same time, maintain focus and attention to the instruction taking place in the classroom. If a student's trips to the bathroom seem excessive, I may have a quick conversation with the student and or contact you.
Believe it or not- students have been known to come up with reasons to get out of their seats so that they can get out of doing their work! Imagine!! :)
