I need sujestions something small I can do during the day.

Walk round the track. Step up and down a step. Or run up and down a series of steps.
But why does it need to be during the day? If you are still at school you are working the shortest hours you ever will. Why not go for a walk/jog after school, while you are winding down from the school day. As an added bonus it will refresh your mind and body and make study in the evening easier.

Good luck.

You could walk around the school during your break/lunch.
Make sure you take gym every year.
Join a sports team, something you're interested in. I played badminton for two years and would lose like 15 lbs a season.

You could also buy a couple exercise videos and do those once you get home from school. An hours workout after school shouldn't mess too much with your schedule.