If I had had to chose a side of the fence to be on with Conner when he started Kindergarten last year, I'd have had to say he wasn't potty trained. He simply had too many accidents on a regular basis, and I had many of the same worries. Many people assured me he'd be fine in school, even his pediatrician. I didn't believe them and was very, very anxious for him.
They were right. He had one accident all year, and it was when he'd gotten a bug and had nasty, nasty diarrhea. When the nurse called me, I was horrified for him. He was sick, but the fact that he crapped his pants clear down to his ankles at school didn't seem to bother him a bit. Go figure.
But anyway, as far as regular toilet use, he just did it. Kids are often very different people at school than what they are at home.