Should i wear my school bag with my cast on my ankle?

If you can: Sure, as long as the school bag is not so weighted down or cumbersome that it poses a fall risk while you have one foot in a cast, than you should be fine. You will be the ultimate judge as to whether you can handle the school bag. Good luck.

Follow instructions: I wouldn't worry about the school bag. If your doctor allows you to put weight on your ankle in the cast then it's probably ok to do so with or without the bag. If your supposed to stay off the ankle then stay off the ankle and use crutches. It may take some practice but you can still wear the bag while using crutches.

Bag is fine: unless there are circumstances that I am not aware of wearing the schoolbag should be just fine - but, as always, it would not hurt to check with your physician for their specific recommendation(s)

Try not to: I think what you mean is that you are wearing an ankle cast due to ankle injury. You are wondering if it is wise to carry a school bag and if this will exacerbate your ankle condition. If your bag had wheels it would be wise to stroll your bag. Alternatively it would be a good idea for your bag to be light. In general a school bag should not weigh more than 20% of body weight.

OK.: It's ok if the added weight does not aggravate the ankle, assuming it's ok with your treating physician.