Technically, they have to live within the school district they attend and play for, unless they have an out of district permit. Out of district permits are handed out like candy, anyone can get them if they say the right things. The problem is that the home lives of many kids in Allegany County are so screwed up many kids legitimately have reasons for attending a different school than where they first start. By extension, the fact that so many people legitimately get them means it's easy to trump up a reason that nobody will investigate or question. Despite anything anyone in an official capacity tells you, many, many kids in Allegany County literally just decide where they want to go and then make it work, officially or unofficially.
There are Fort Hill kids living down the street from Allegany and Allegany kids living down the street from Fort Hill, and it's just not football, it's every activity.
That being said, it can happen in Mineral County too, it's not as common because Keyser and Frankfort are pretty far from each other, but it has still happened . If they were in the same city I think you would see it there too.