The 75-300 isn't going to do it for night use on a typical high school field where the lighting level isn't great. The 70-200 f2.8 is my most used lens for HS football.
Minimum shutter speed for HS football is 1/800 but you can drop to 1/640 if you have to but it will start inducing significant motion blur with a lot of the action.
When you get the 70-200 f2.8, set your camera to manual exposure with ISO set to auto. Set the aperture wide open (f2.8) and try the shutter speed at 1/800 and if the resulting ISO is within reason for the noise of the T7 then leave it there. If not then try going down to 1/640 if you have to; 1/500 is at the point where you will see a lot of blur on fast action so except for special situations don't go that slow or slower during football.
Shoot RAW instead of JPG so you don't have to worry about white balance while shooting and you can also clean up a lot of noise in post when shooting in RAW.
I set AF to servo AF and drive to high speed; I use a single focus point which gives me best control of what the camera is focusing upon and with your T7 which has a slower focusing algorithm than my 1DX series it also transfers some of the workload from the camera to you. Using multiple focus points will often slow focus acquisition slightly and could cause you to miss the "killer" shot you want.
Even with drive mode set to high speed, do NOT rely upon bursts too much. Timing is still critical even with very high frame rates.
Some of the great photos in sports come from photos on the sidelines or in an end zone celebration after a TD. In those cases, you are shooting a group that is not moving as fast so you need to close down the aperture (higher f number) for greater depth of field while simultaneously slowing shutter speed to keep the ISO within reason. The 1DX series bodies I use have a handy button to toggle between two sets of exposure settings but you can achieve the same result by manually changing the aperture and shutter for that quick celebration capture but do NOT forget to change back. Get in the habit of looking at the exposure info in the viewfinder so you will quickly catch any mistake. I lock the controls on my camera bodies once I dial in the setup for the game to avoid accidental changes as I switch between two or three bodies.
Most importantly, it is a privilege to be on the sidelines and you have to always be alert to protect the players, officials, and yourself. Don't try to review images during the game except maybe a brief check during a time out or between quarters and halves. Many times I have had to run back out of the way and a few times I had to jump over players crashing well over the sidelines. NEVER get in the way of an official and respect the privacy of the coaches. If there is an intense exchange between a coach and player or coach and official, do NOT be there clicking away; have respect for those who are truly part of the game. I have been shooting football for many years now and the area officials know me and know that I know what I am doing. Since you are starting out, make a point of catching the officials during some "dead" time before the game starts. Say hello and ask them if they have any special instructions/concerns. I shot a soccer match last week and was talking with one of the officials I have known for years. He had a big bruise on his face and it came from a clueless photographer who caught him with a telephoto lens as the official was running down the sidelines to follow the play. If the photographer hadn't been in an area where he didn't belong, that wouldn't have happened.
You will love the 70-200 f2.8! (and don't forget to grab some sideline photos during the game, there are great possibilities for high emotion shots!)
Rodger
wow those pics are amazing
here are a few of mine that need lots of work compared to yours
