keep feeling tired in school and i have exams?

ive just got back to school after a week off of school and ever since ive been very tired at school no matter what! i have to lay down on the desk and close my eyes for like 10 secs. ive tried going to bed at 8 30pm and no difference! plzz i need help becasue i cant go to bed earlier cause i need to revise for exams and i have an exam tomorrow so i CAN NOT be like this during the exam becasue currently i cant concentrate in school or anything, i cant seem to look at the teacher and listen for more than 5 mins so i have learned nothing which means i have to revise more! plzz someone help me. i cant have energy drinks cause both my parents and school say no to them, ive had it like once in my life thats it.

PS i wake up at 7am so going to bed at half 8 and sleep at 9 means i had 10 hours sleep with no difference, still tired......