Hi Apricot
What about thinking if the worst did happen assuming you "had to go" you would probably pretty much automatically go into the school and either use a toilet or ask to use one! No one else would think anything of it! Try and think what you would think if you were someone else looking at the situation.
What about next time u go to school to pick them up wether u need to go or not go and ask to use the toilet? Just so you know any time you need to you can.
Just some ideas not sure if they are any use or not to you!
Take it easy and best of luck for the new term!
Matt