At our school, we have two-a-days from 7-930, a 30 minute break, and from 10-1230.

What should I eat during the break?

eat a banana , 90-100 calories :]

or any fruit

about 60-90 calories. very healthy to eat fruits. 5 servings a day, makes the doctor go away!

add some protein which you should have with every meal anyway

yes, protein and fruit to stay in positive nitrogen balance.

if you're a lean guy already, I would add some dextrose to the shake too, and limit it to 1 piece of fruit. dextrose gets into the muscle glycogen storage faster than fructose (in fruit) since it doesn't need to be processed by the liver.