Night-time action is the toughest conditions to shoot in but luckily, modern DSLRs are the best tools for this.

You will have to stack the odd on your side to get a reasonable rate of keepers by:

• Using a high ISO. It is safe to use ISO 800 on all current DSLRs for even large prints but you can get away with higher sensitivities, depending on your intended print size. Take a few sample shots in advance to see what each ISO looks like.
• Use a high frame-rate in continuous drive.
• Leave the flash off. Not only it will extremely rarely reach the players, it will slow down your frame-rate and - if it ever reaches - it will bother players.
• Use continuous autofocus ('AI Servo AF' on Canon) or practice manual focusing a lot.
• The shutter-speed should be high (1/1000s or faster) to freeze action which the high ISO will let you do but you will also need to open up your aperture, probably to its maximum F/2.8.

Buy or rent a longer lens. My wife employs 3 sports photographers who cover football regularly and even with their privileged location, 70mm is rarely enough reach to capture worthy shots. The tool of choice is the Canon EF 70-200mm F/2.8L. No need to pay extra for the stabilized version as you need fast shutter-speeds.