Do not use an undefined acronym in a college application essay.
It is okay to use an acronym once it has been defined in the writing. For example: I play on a team sanctioned by the American Youth Soccer Organization (AYSO).
Once defined, then it is fine to use that particular acronym in the same writing.
P.S. “NFL football game” is not necessarily redundant as there are several professional and several amateur football leagues. Also, some refer to soccer as football so any further clarification might be helpful.
Which raises the question: Is it ever fine to use an undefined acronym in a writing. The answer is probably yes as some acronyms are more widely known than others. “NFL” is so widely known in the US that defining it is probably not necessary.
FWIW The word “football” could be redundant if one stated: “Let’s go to an NFL football game.” But neither “NFL” or “football” would be redundant in other situations. Regardless, the explanations are beginning to seem redundant.
Also, whether or not using an undefined acronym is okay may depend upon your target audience. If communicating with a group involved with youth soccer, then using AYSO without a prior explanation is probably fine.