Just use a sticky note to write down the main parts of your dreams and try to go over it as much as possible in your head right after you wake up so you don't forget. After school when you have some time, you can look back at the post-it(s) and it will jog your memory and hopefully bring back any more details you remembered.

Either that, or you can keep a tape recorder by your bed and just talk into it as you're getting ready to leave for school. Then listen to it later and record your ideas onto paper.

Hope this helps