Infant teacher here:
On the practical side, it REALLY makes life easier if the child has a clean change of clothes, trousers, pants and socks, in their book bag or PE bags, which the teacher and teaching assistant know about.
It means that the child can be quickly sorted out, into clothes that fit, without the teacher having to scrabble around in the spares box for clothes that usually don't fit too well!
As for having accidents most days... It's not uncommon. Although you may find that the teacher suggests taking him to the doctors to get checked out, just in case its something more.
The school should be encouraging him to go at every transition time, like before assembly, before lunch, after lunch.
It might get to the stage though where he needs something a little more personal, like a chart. Every day that he manages without an accident, he gets a special sticker, leading up to a bigger reward. I've done that with Receptions and it works well. You could run that from home if you wanted. A big fuss if he comes home without a plastic bag of wet stuff, and an "Oh well, never mind", if he does. You say he was dry before he started school, so he's clearly capable of being dry, he just needs a little encouragement. ;)