camera shake, do not be afraid to use a monopod. shoot from your knees.
all of the Shutter speed advice is good. Plus - remember - you are shooting FRESHMAN who are really not as fast and hard hitting as the varsity. It gets fun! Sooner or later you will get a dark day or night game and your lens will really challenge you with motion blur. Your pic's here are quite good. on the contrary, I also always have trouble with the blown out white on bright days. (i.e. your helmets).
As for shooting football, it is actually a simple sport to shoot, in my opinion, baring the ref's butt in your way and multiple lineman. Soccer Kills me! Know the plays. Start with the QB if you want to shoot the play. Position yourself 15-20 yards ahead of the team you shooting. (you will have no ref in your way, nor any team players/coaches).
Follow the QB withe focus and keep you other eye OPEN and on the RB's. You will learn to see the QB's hand for pass or run. If he is passing, you can always shift the camera to a receiver and defensive-back, while looking where the QB is looking. (if that makes any sense.)
For shooting defensive players, reverse your position on the sidelines and be 10-15 yards behind the play.
Night football is rough, as you can tell by all the many posts here on night football, flash & ISO!!!
Enjoy! Keep up the good work. And just when you get it right ... you'll go inside for basketball/wrestling/swimming, etc!!