In the United States your kid cannot go to school if they aren't potty trained unless they are attending preschool and are working on getting potty trained.
Many preschool kids go to preschool and are not potty trained but are working on potty training and some preschool kids need diapers and others wear pull-ups.
In the United States the schools require kids to be potty trained when they enter Kindergarten or they won't be accepted into or enrolled into school unless they have incontinence or a disability that requires them to wear diapers.
My nephew went to school until he was 8 years old and was not potty trained but the school didn't know he wasn't potty trained.
He would hold his poop in until he got home and then poop the diaper but he would pee the diaper through the day.
The school didn't know about the diapers nor did anyone else and he only needed diaper changes at home.
In other countries a child can attend school not potty trained and some as old as 11 years old have been showing up to school in diapers and not potty trained but it's not the norm in the United States.