I would honestly be a lot more concerned about support for a possibly gender non-conforming kid than whether the school is challenging enough. In not-so-challenging school, the kid can accelerate (skip grades or attend a grade or two ahead in specific areas), take online courses, follow their own interests with outside and family support.

But if the school and community are not supportive of queer and trans kids, then that can have a severe impact on their mental and physical health.