Well Hobby Photo user mom, first off you may want to start your own thread, This one is years old. You will get more advice in your own"new" thread. However, that said let's talk. The &D Mk II is a fantastic camera and Lightroom is top of the pile for post editors. So, you are off to a good start.
Most of the time the camera is simply a storage device that stores what the lens sees. Your lenses are letting you down for nighttime football. Especially if it is PeeWee or high school fields where lighting may be less than spectacular. The most important thing to remember in sports photography is location. Not camera. Not lenses, it is where you shoot from that is paramount. Shooting from the bleachers and you are not likely to get Sports Illustrated quality photos. Not even if you have the top of the line Canon gear! Sidelines are best.
Now for the bad news the Tamron 70-300mm f/4-5.6 is a poor lens. It will never be up to the great IQ photos most people want. The EF-S 55-250mm f/4-5.6 IS STM is a good lens actually one of my favorite crop camera lenses but, it also, is not up to night time sports action as a first choice. But you use what you have, right?
As to how to set up the 7D Mk II, it works exactly the same way your Rebel did. However you set it, you can do the same for the 7D2. My preferred settings, to start, would be using Av mode and set the lens to its widest open aperture. Set ISO to 1600 and use AWB. Actually the WB doesn't matter since you will be using Raw file format. Never, never jpg. You have LR so the file conversion will be seamless and automatic when you import into LR. There you can change most settings to your liking.
The one thing you can't fix in Raw is focus. That must be spot on. Use just the center focus point and turn all the others off.
Give this a try and, please, let me know how it went.
If you need a few pointers on LR feel free to ask. I have a few years under my belt with it and PS. 
Your 7D2 will do a pretty good job with high school football but your settings have to be correct. The lenses you have will work fine with the 7D2 but they aren't optimal for a typical high school field where lighting isn't great.
For football, I like to stay at 1/640 as a minimum shutter speed and for many fields at night I have to shoot at that minimum. I am using f2.8 lenses and with 1/640 shutter speed that typically results in ISO 10,000 or slightly higher at many high school night football games. With your 70-300 and 55-250 lenses, if you used the same 1/640 shutter speed at this field your ISO would be at 20,000 at the wide angle end of your lens where it is a f4 lens and 40,000 at the telephoto end. IF you select the expanded ISO option on your 7D2, it will go to ISO 51,200 but I suspect it is so noisy at the high end of the range that the images won't be very good.
With the lenses you are using, I would do the following:
1. Choose the best location that you can and with many high school stadiums the visitors' side has closer field access than the home team. Get as close as you can so that you can stay near the wide angle end of your lens more often. I have sidelines access which makes it much easier but I have advised parents to consider using the visitors side for photography no matter whether they are home or visiting because they can often get closer that way and in many cases there won't be many fans, cheerleaders, band members etc. on that side of the field.
2. My preference for sports is to set the camera to manual mode and set your shutter speed and aperture to fixed values with ISO set to auto so that it can vary for a correct exposure.
I don't own any lenses that vary aperture by the "zoom" level so someone that does will have to fill in what happens to a fixed aperture exposure setting when the lens is "zoomed" past that level. The camera aperture will change to follow what the lens provides when you go to the telephoto end but I don't know whether it will automatically return to the wide open setting when you go back to the wide angle end, I suspect that you may have to manually go back to f4 after zooming to the tele end.
However, what you are going to need to do is shoot with the aperture at its wide open setting for whatever it is at a particular focal length and then adjust your shutter speed downward to the point where you get an acceptably low ISO for the quality that you need. Some shots in high school sports can be taken at 1/250 or lower such as when the QB is frozen before throwing HOWEVER be careful that your shutter speed is fast enough that you are able to hold the camera steady enough to avoid blur from camera shake. Typically that would be around 1/250 to 1/500 at the telephoto end of the lenses you own and significantly less at the wide angle end. Of course you also have a wider aperture at the wide angle end and this is again why you want to be as close as possible.
3. I don't think either of those lenses you currently own are known for extremely fast focus and the aperture also slows down the camera AF mechanism so you need to be aimed at what you want well prior to image capture so that the camera AF system has a decent shot at achieving focus. I prefer to work with a single focus point which can help to speed focus acquisition but you may want to use a small group of focal points to make aim less critical. You want your focus points on high contrast area such as the numbers on the jersey.
In short, shoot in manual mode with the widest aperture possible with ISO set to auto and adjust shutter speed downward as needed to get a reasonable exposure. A noisy shot is preferable to a blurred shot and you can do reasonable noise cleanup in post while a blurry shot will always be blurry and virtually useles except for the rare good creative shot. So accept high ISO noise over too slow of a shutter speed. 1/500 is marginal for football action and anything below that is going to be blurry except for stationary images.
Most high school fields have very uneven lighting so your quality rate will go up if you concentrate on the best lit part of the field and for most high school fields this will either be a zone around the 50 or dual zones around the 35-40 on either end of the field. End zones are often poorly lit and many just barely meet applicable standards (and often don't because of missing/defective bulbs in some of the light stands).
Your 7D2 is a fine camera for shooting sports and besides gaining knowledge and experience, your single biggest gain in performance would be from a f2.8 lens of reasonable telephoto length. The Canon 70-200 f2.8 family of high quality fast zooms is a go to lens for sports and if needed to hit your price point, image stabilization isn't an absolute necessity for sports because you are going to be shooting at a shutter speed where it isn't needed most of the time. You can also consider buying used and at that point IS might be a minor drawback because it is one more thing to go wrong with a lens although the Canon IS system seems quite robust. I shoot with two 1DX series DSLR bodies at most events and one will almost always have an EF 70-200 f2.8 on it.
Ernie has been trying to convince me to try a Sigma 120-300 f2.8 zoom for several years and that might be an excellent choice for your 7D2, it would be a huge step up from the slower glass you are using now and will work well if you are a ways back from the sideline.
Shooting high school sports is challenging but fun and you are always going to be dealing with marginal lighting except with the rare exception of a well funded private or large city school. Choosing the best location and concentrating on the shots that you can get are key to getting good photos. Don't neglect to get shots during the pre-game warmup because often there is enough natural light then to make slower lenses less of a drawback.
And accept that the photos you get aren't going to be like photos taken during the daylight or in a pro facility. The image below is typical of the lousy lighting for high school fields, shot at 1/640 with a Canon 1DX II and EF 300 f2.8 prime wide open and it still required ISO 10,000 with the shadows heavily pushed during post. This year, high school football has been pushed to an early March start and maybe the reflective snow on the fields will improve the lighting 🙂
Rodger
