Personally, I haven't had to really miss any school/class because of my diabetes. I've only been sick with ketones once (besides at diagnosis), and that was just a little over a month ago, when school wasn't in session.
As far as low blood sugars go, I always carry my purse with me to classes, so I have my meter and glucose tabs or some other low food with me. This way I don't have to leave class to treat a low. And I also always have a water bottle with me, anyway. So if I happen to have a high BS then I already have water.
So, no, I haven't had any problems with my friends and teachers at school. Almost all of my teachers are fine with having to check or treat in class. Ironically, I don't think I've ever had a time when I had to check or treat in a class with a teacher who I didn't like. Whenever I've had to, I've been in a class with a teacher who I know well enough to do it without even asking. Now that I come to think of it, I think the class where I've had to do most of my diabetic things is newspaper. I've developed such a close relationship with the newspaper advisor that I would consider her more of a friend than a teacher, so she obviously doesn't care if I do anything in that class. Although we'll see how that changes this year, because that advisor just retired. )=
And the same goes for the newspaper staff- we're all so close anyway, that they all know about my diabetes and don't even really notice when I have to do something. (=