Wear clothes which let your armpits breathe, light fabrics which don't cling to your underarms and cause them to get warm and wider sleeves or sleeveless ones.
Maybe difficult if you have a set uniform but see if you could get away with a sleeveless school shirt.
Keep the underarms shaved and as clean as possible (antibacterial washes are good like Sanex or Carex).
A bit of unperfumed talc on a big cheek brush can also give extra confidence.
The Mitchum antiperspirant deodorants are very good, esprcially the roll on which you could follow with a spray (from a distance) of an unperfumed antiperpirant like Sure. Some of the sports ones are stronger and more body responsive.
Make sure you note the difference between an antiperspirent (stops you perspiring/sweating so much) and a deodorant (covers odour) on what you buy.
There are stronger ones like Driclor but they can be quite harsh on your underarms (you can ask a pharmacist in somewhere like Boots about them).
Keep your feet cool as that will tend to keep the rest of your body cooler.
You could take some wet wipes in your bag for a freshen up, again, avoid perfumed ones. You can also get little bottles of antiseptic hand gel which may help in a freshen up.
It is the bacteria in sweat which smells so avoid anthing too perfumed else the smells can mingle and make it feel worse.
You can use a body spray fto smell nice, just not under your arms.
Hope this helps :) x