I had a few roommates at BYU that tutored or were TA's for BYU football players.

In one case there was a player on the team that had transferred in from another school the year before. After a midterm the player emailed my roommate who was the TA asking when his test score would be changed. My roommate let him know that since he literally didn't take the test he would get a zero. The player then complained that at his previous school they didn't have to take tests and the TAs would put in a grade. 2nd hand account on my end but I believed it.

At least a few years ago probably the most common football team major was "Geography". The major is only 40 credits(the average is probably double that) to complete and generally easy.

I had a summer class once with a bunch of football players, including one very high profile guy. The professor would had back tests and assignments back in one pile so you would end up seeing everyone's grade while you looked for yours. This high profile player in particular did absolutely terrible on every assignment I saw including a big term paper that was worth much of our grade. No doubt he should have failed the class. Luckily towards the end of the term I heard him talking with the professor before class and professor was working with him to bump his grade up to a C.

Long story short, grade manipulation for college athletes is certainly very common.