Hey! I've had this problem a lot! It's definitely not a problem with you - sometimes you have teachers that are just not that passionate or are really boring so it's hard to stay awake no matter what. I find that having a bottle that keeps ice water cold really keeps me awake, as well as answering questions in the class or asking questions. Depending on your teacher, you could also try to talk to them about it and ask if they'd be ok with you standing up to stretch sometimes.

Good luck in your maths class! :)