As said above you just have to re-fix your sleeping patterns, I remember last year I was absolutelty wrecked after the first week of school, since I tried to shift my sleeping pattern by 4 hours in one night. Once you get back into the swing of things, you won't have time to feel tired, the year will fly by. Try to start going to sleep earlier and stick to a certain time.
Have to add that in most of my classes those who actually got a good sleep were the ones who performed better during school (although this could be more to do with how disciplined you are with yourself). And as said by a lot of people above ENERGY DRINKS ARE NOT GOING TO DO YOU ANY GOOD, if anything they are going to have the opposite effect (relative to education/school e.t.c)
So don't worry, you will work back into the routine and you won't feel tired (for now, by the end of the year is a different story, relative to burning out e.t.c) but for now, just try to get back into the pattern.