So you've managed to pigeon hole the problem to 'school shootings' when the vast majority of murders are not school shootings. And this is all theorized under the assumption that there is no black market for semi-automatic rifles or they cannot make straw purchases. Someone who plans to commit mass murder doesn't care how difficult you make it. If there's a will there's a way. The only effect your legislation will have is to disenfranchise the vast majority of under 25 who aren't violent and criminally inclined.