It may sound a bit silly, but you could try taking a bottle of water to all your lessons. The weather's really hot at the moment, and that could be making you feel faint (well, exagerating any problems you may have been having before). If you drink a bit whenever you start to feel a bit off, you might find it helps.
If you think it's stress, though, you could try getting something that smells quite strongly. That should bring you back if you start to get faint, and if you start to be able to get through the fainting patches, then the anxiety surrounding them should decrease. Perhaps lavender? That's good for calming down. Or, I think lemon is supposed to be good for waking you up, which might be useful.
And, uh, it sounds obvious - but eat plenty of food (keep your blood sugar levels up), and stand up slowly whenever you get up. If you feel faint, sit down immediatly - it's better to look a bit stupid by sitting down in a weird place than to end up falling down the stairs or something.