I photographed a 5 a side tournament a few weeks ago, 14 to 18 yr olds. It was something we had organised and we got all parents, kids etc to sign disclaimers in case we wanted to publish any photos that would be taken.
A couple of parents then mumbled to my colleagues seemingly not too happy about me taking the photos ( no one approached me, and I was all over the place with my official name badge, D200 and 70-200 f2.8 Not something you can hide!
They were told I was the official photographer and reminded they had all signed the disclaimer. Smiles all round then, they were happy.
I suggest you get approval from the school, then either have a PRESS card tucked into the band on your hat or get a badge made up that says " official photographer" Seems all they want is confirmation that you are ok, and the confirmation word "official" somewhere on your person dispels all fears.
Good luck
Allan