Ask someone if he can give you an aspirin. If there are no schoolnurses, the secretaries often have one in their desk.
If you are not at school right now take a long nap, your body will need the energy if you are ill.
If you really can`t take it anymore, ask your teacher if you could go home because you`re feeling ill and need some rest and tell him you will come back in the evening to hold that presentation.
And if you are feeling dizzy keep away from computerscreens, they will only make it worse