I have a twelve year old boy too. I remember that whenever he was injured the school would contact me, I think that's required by"law", because he is a minor. O.K. they have given him first aid and assesed his state as probably o.k.to stay in school for the rest of the day, but you should have been contacted and asked for opinion whether you will pick him up or leave for the rest of the day.
Another important thing...MAKE SURE you ask to see "accident book" they should have filled in on the day of the accident. Accident book is important for the future, if your boy's injuries turn into something nasty in the futute, all kids are inssured on the school premises, as well as the staff and visitors, possible compensation in the future.