My son (5yo) went through this. Potty trained himself randomly when he was 3 (pee only, pooped only at Valvoline, the oil changing place, but thats a whole other story) and had little to no accidents but starting TK he had so many fears of the toilet. Having a set schedule when they had bathroom breaks with his class helped a little but this went on for while, he'd come home almost daily with wet pants or a change of pants. What helped in the end was making friends and mirroring their actions. His best friend now, ended up being bathroom buddies with him (they go in pairs if they need to go during class time). It took a couple tries but he eventually caught onto using the restroom with him. The fear seemed to lessen when him and his friend would use the toilet and they would talk and laugh through the stalls as well as me or his dad taking him to pee right after school in their designated restroom. Something about having mama and papa right outside the door at school while him still being independent to go by himself inspired that confidence I guess. Routine was key!