Bullseye! I'd like to add (because Title IX has been thrown like a grenade) that if the practice field can be lined off to meet the "standard dimensions" of TSSAA soccer fields, and the football field can't for whatever reason, they could be forced to use the practice field using Title IX against them (hint,hint).
You hometowners need to follow BPM's advice and support the kids... all football programs in this area are funded by a collusion of sources, and boosters work alongside coaches for the betterment of facilities. I doubt that it would be readily accepted for another program to enjoy the fruits of football's labor without reciprocating upkeep at any school, and rightly so. You Title Nine'rs have a case, but it's not with football, it's with the central office.
Edit...
For the "taxpayers fund all sports..." comment. Sports are not a requirement. Title IX ensures EQUAL opportunity for both sexes, nothing more or less, and the public portion (taxes) of financial support is determined solely by administration. If any soccer booster feels entitled to the use of another program's designated facility "just because it's there", you're wrong on every level.