I've worn mine for upwards of 12 hours before. I'd be sore, but it wasn't unbearable.
In the mornings, I'd recommend getting dressed last. Do everything else you need to do to get out the door, and then get dressed. It'll give you a little more time out of the binder.
At school, take a break whenever one is offered and go to the bathroom to stretch and re-situate. Same with extra-curriculars. Don't be late to anything, but take as much time as is available to give yourself some extra comfort.
As far as alternatives, if you're fairly flat, they do make some pretty hardcore sports bras these days. Go look at a running store. That would probably get you fairly close to flat. Layering a sports bra with a tank top underneath another shirt would probably work. Tucking in the tank top would enable you to get pretty flat.