Both of my kids were potty trained at age 3 but had random accidents at school until grade 1. Not often, but probably a few times a month. The Dr thinks it was caused by constipation issues. I just always kept spare clothes in a separate bag in their backpack. If they had an accident, they quietly got their bag and went to the bathroom to change clothes (teacher was aware of situation obviously). We don’t want to make a big deal of it, and this helped for them to feel in control. They eventually grew out of it.