The problem with your logic is that you knew they joined.  I assume it is because they told you.  But honestly, what self-respecting parent doesn't know the clubs their kids are a part of.

This really should be a non-issue, because everyone at the school would already know who is in that club, including those kids' parents.  So it is confusing why Kenney would have suggested "outing" kids in GSA's and it is confusing why there would need to be protections for hiding the kids in them.

If you join a GSA, gay or straight, everyone will know anyway.  End of story.