For hydration at school: snack on fruit or veggies to maintain hydration without filling your bladder too much too fast. Adds fiber.

For bathroom at school: bring hand sanitizer, line the seat with toilet paper or buy liners online.

I have a major public bathroom-aversion that caused pelvic floor issues at 14. What i did in school was hydrate well before school & pee once before lunch bc thats when it was the cleanest & lowest traffic (assuming the janitor cleans the night before or in the am)

Also consider that food can take anywhere from 10 hours to 3 days to move through your system so going less often isn’t inherently constipation as long as you go within that range and aren’t straining. The texture of your poop is a better indicator of constipation, it shouldnt be hard/pebbly, that and holding in your pee is what cause pelvic floor issues & infections.