Potty advice

My 3 year old is starting school in fall and he uses potty at home but at times struggles putting pants back on. In public we mostly out on a diaper since not all places have bathroom but if they do we take him to the bathroom and see if he needs to use it but I do how it’s going to be when he starts school. I’m not sure how to teach him that in school he needs to ask to use it and what to do if in case he needs assistance with like putting clothes back on