Whoever wrote this has never played a brass instrument or sat anywhere near someone playing one. (cc @BigAlStrood)

They haven't thought this one through either. A child might be fine with a 20-minute lesson out of doors in January, but the teacher is doing 20-minute lessons all day long. Also, temperature affects the pitch of brass and woodwind instruments. And metal can get very cold....

And they've obviously never seen a school practice room.

I might as well give up. This advice undermines everything I teach.