[Born in a Rage]

If the act of paying school taxes somehow entitles one to use whatever school facilities/programs he or she wishes, then the school would have no right to exclude adults from their band, either.

No, because schools are for people of certain ages.

It's more like having a public senior center that is paid for by taxpayers tell a senior that they can't play with the senior citizen band because they play bingo with the local church (who is the senior groups competition).

[Alberta's Child]

I don't think your example is a good parallel. A senior citizen can go to one center, fifteen different ones, or none at all. A child's attendance at school is mandatory in most states, and a child cannot be officially registered in multiple school districts.