This isn't a feasible solution.

It's very unfair to your parents to have a variable start and end time to your practices on a daily basis.

It prevents your students from being involved in any other activities outside of marching band.

It prevents your kids from working.

It prevents your parents and kids families from doing anything for a full extra hour of their afternoons.

If this was your solution, I strongly urge you to find another one. Rehearse before school. Start fundraising for a series of tarps/tents to put a sun shield on a part of your practice facility.

What you chose to do is frankly abusive to your community.