The short answer is NO. You do not need permission as long as your stated intent is just how you described.
The complex answer is (consult a lawyer for legal definition), common courtesy will at minimum have you let the organizer - principal - coach - anyone you think should be told, you are there as a freelance photographer covering the event. In this overly sensitive age (with every right to be!), photography is on the verge of a black cloud art or profession. An increasing number of people are becoming suspicious of individuals with any camera that resembles something more than a regular store bought point and shoot.
If you are approached, do not cause conflict, do not state "this is my right" kind of lines, be polite and if a parent asks you to not photograph their child, simply oblige. Chances are, they will eventually look at your work and change their mind at future events.
If an organizer approaches you, follow the same tactic above. If you are approached, chances are you have done something to draw their attention (been in an area you shouldn't have been, complaints from athletes if your flash affects them, etc). This paragraph alone could spark an entire "photographers rights thread", I dont want to get into. Just be courteous and ask orghanizer's, etc, if you can be there.
If you are approached by another photographer who is there for the same reason and you have gained permission to be there, get their business card, best know what your competition is. Now, if they are hostile towards you, you have no choice but to duke it out to the death! (i'm kidding).
If your intent is to photograph a person or persons for commercial intent (advertising a consumable item with a recognisable face/person), you not not only need permission, but also a model release).
I have the shortest fuse in history, but as a working freelance photographer, I have quickly realized that displaying aggressive gorilla tactics to people/persons who are simply asking what I am doing, will get me nowhere, when it comes to selling prints.