It is my understanding (OPINION!, not legal advice.) that the only time the grounds are off limits is when an "activity sponsored by a school or educational institution is being conducted". I carry at the local HS track on the weekend, as long as there are no teams/bands practicing there or no other SCHOOL events taking place. A local league / non-school soccer practice is not a school event, IMO.
TX PC 46.03 http://codes.findlaw.com/tx/penal-code/ ... 46-03.html
(a) A person commits an offense if the person intentionally, knowingly, or recklessly possesses or goes with a firearm, illegal knife, club, or prohibited weapon listed in Section 46.05(a):
(1) on the physical premises of a school or educational institution, any grounds or building on which an activity sponsored by a school or educational institution is being conducted, or a passenger transportation vehicle of a school or educational institution, whether the school or educational institution is public or private .................