Be prepared. I don't know statistics on the actual frequency, but the best plan is to make sure you are prepared. Ask your mom when she got her first period- you will likely start close to the same age. Keep some pads and an extra set of underwear and pants or a skirt at school. If you feel you are close to starting (a clear white discharge often appears about 6 months prior) wear pantiliners for protection.
No numbers!c. Certainly this happens with some frequency but i know of no statistics. It is an embarrassing situation that should be handled with the utmost compassion and assistance to the young girl. Education and preparation can prevent this from happening. The school should have a backup plan to help girls and a phone call to a parent, relative or close friend can assist. Be understanding and compassion.