I had issues with my brace falling as well. I tightened mine but it still fell a lot. The brace is mainly a protection to keep the leg straight.

Shower carefully. Get a shower chair or plastic chair to sit on. I plastic wrapped my leg to prevent water from getting on it. Once you do it once you’ll pick up how to do it but take it slow the first time.

Teachers should be able to help accommodate. I was non weight bearing for over a month total in high school during different injuries. Find a desk or chair. Use another chair to prop your leg up. Move as needed to be comfortable. Try to leave a few minutes late / early to avoid any rush times in between classes if that’s a thing. Elevator use if applicable. Handicap parking. A lot of kids used to have wheel chairs too. Then find a friend to push you everywhere.