At my school your planner/assignment notebook/organizer (whatever you like to call it) that is given to you at the beginning of the year is your pass. In the back of it there are like four pages just for getting it signed so you can leave. There is also a smaller version of this on the bottom of each day; ask to leave on May 5th, have it open to May 5th.
You have to write where you are going and what time it is then you have to ask for the teachers initials. That is the general rule, and each teacher adapts it to their needs. My chemistry and world history teachers would tell you just to take the little book with you if you were going to your locker for your textbook. Some teachers tell you to take it with you if class hasn't started yet so that you aren't swept.
Personally I think it would be nice if you didn't have to ask, but it's really not that bad. And if you really have to leave, for like an emergency, like I had once (I was basically passing out), you can just get up and go. I can handle that.