For UIL, the rules should be here.
http://www.uiltexas.org/files/constitut ... n-1478.pdf
There is no state-wide organization that administers eligibility for private school junior high athletic eligibility. In many metropolitan areas, there are leagues which have formed that have varying levels of rules and discussions about same.
For example, in the San Antonio area, I am aware of at least four such groups: Archdiocesan Interscholastic Athletic League (AIAL) for Catholic Schools, Christian Athletic League of San Antonio (CALSA), primarily the smaller non-Catholic private schools and the Independent Schools Athletic League (ISAL) for the larger private schools. There is also a six-man league, the South Texas Football Conference (STFC) which may include members of each of those leagues.
From the leagues where I have been involved (CALSA and STFC), the general rule is that students must be enrolled in 6th, 7th, and 8th grades (no 9th grade students), have a maximum of 3 consecutive years to compete starting with 6th grade enrollment and be no older than 15 as of September 1 (we took the 19 age for high school and went back four years).
I would seriously suggest any junior high league that forms spend a couple minutes drawing up general eligibility rules. Doesn't have to be Tolstoy's War & Peace length, but you'd better make it clear. It's amazing how many times that "little ninth grader" slips by and what a shock ... he's the athlete of the year for the league.
Oh yeah, and when you draw up those rules, get the coach, athletic director, principal and the pastor to sign off on them. Equally amazing how often you would hear that gosh, oh, golly gee, ol' Fred who ain't here no more musta agreed to these and none of us, as pure as the wind-driven snow, would ever willingly break the rules. And by the way, it's for the kids ... especially my kid who is hands-down the best athlete in the (insert name of city, county, state, nation or planet here).
It does happen. I remember sitting in on a meeting for a league we were thinking of joining. Next item of business was a question of eligibility. Seems one of the schools had played a kid in junior high for five years. The school gave its report, and basically said, yeah, we messed up, it was wrong, and threw themselves on the league's mercy for punishment. Another representative from another school asked, "Is that Pastor 'x's' son?" The school said yes. And that was the end of the discussion. It took me three hours to get my jaw back up into its socket from hitting the floor.