For me, backpacks are pretty much the only thing I would ever wear for 6th form come September because:
1) I cannot tackle a shoulder bag
2) I cannot imagine being able to tackle a shoulder bag
3 I will never be able to tackle a shoulder bag
My advice is go for a backpack if that's what you want, and especially if it's associated with your health too. Put aside what others may think of you, those that judge you for what you're wearing are going to be people you don't really want to get involved with anyway. Therefore they are irrelevant, and shouldn't be part of what you choices you make.
So, do what you want and screw anyone that tells you otherwise.