If you're leading it OFF campus without ANY school resources or input or doing any work for it AT ALL at school then yes, it's an independent group. If you're using ANY resources for it in ANY way without compensating the school or district then it falls under your purview as band director and is a part of your program, thus applying TEA laws.
This could even be as simple as "the group meets in the band hall on Thursday evenings" without the group RENTING the space from the district. Or as simple as "I provided flag poles from my color guard that we don't use anymore" without them being disposed of through proper district channels. Both are a misappropriation of resources and could get you in hot water on their own.
Just be careful. Plan on 8-hour rule being in effect during the school week, and it being open on the weekends, like state law requires.