I agree they should be given a week between seasons. Do they really have to wait until the state championships have been played even if their team lost in the first round of the playoffs? That makes no sense to me.
Seems to me like it should be up to the school's AD but I can't fathom an AD that would allow a kid to practice with next season's team regardless of the sport. ADs should insist on the kids only practicing with the current season's sport.
I have no problem with lifting weights or conditioning with next season's team as long as the current season's coach allows it.