I respectfully disagree.
The D90 kit lens (18-105mm f/3.5-5.6) is probably not going to give you the results you want. Even worse news is...sports photography is just about the most expensive thing you can get into. You can easily outspend what you paid for your camera for a good quality zoom lens geared towards sports. Some thoughts...
1) You're almost certainly going to need a tripod or monopod. More often than not, you're going to be at high zoom levels, so steadying your shots with some support is a must, otherwise they're going to come out blurry.
2) Plan on getting out on the field to shoot from the sidelines. You might need to talk to some people in order to make sure this is okay.
3) Most H.S. football games go on into the evening hours when it's dark. Stadium lights will help with your lighting needs, but there will be those odd times when the lights haven't come on yet and you need more light.
4) I would be leery about using flash, especially if you're on the field.
Football isn't "fast" action like MMX racing, so you don't need insane shutter speeds (like the 1/1000 I was using last weekend) but even still, 1/640 is about as low as I would venture for stopping action. Because of twilight/night shooting conditions, you need to run your aperture wide open to let in as much light as possible so that you don't need to crank up ISO settings and get grainy photos.
I think the aforementioned 70-300mm lens would be a much better choice than the kit lens.