Personally I can't see anyone handing in a $3,000 weapon, they would just 'lose' it.
They also tried the gun control thing in Australia.
Latest research on the issue concludes: "
The hypothesis that the removal of a large number of firearms owned by civilians [would lead to fewer gun-related deaths] is not borne out by the evidence."
http://www.time.com/time/world/article/0,8599,1736501,00.html
I don't think that means a great deal myself, because the cultures are different.
Actually if you really want to kill lots of schoolchildren, try dynamite:
http://en.wikipedia.org/wiki/Bath_School_disaster
(or Tannerite can be bought online in the US if you failed Chemistry O-level and can't make your own explosives).
Incidentally (to those mocking the idea earlier with hilarious memes

but who don't have the balls to reply), the Israelis tried arming school personnel after the Ma’a lot school massacre in 1974, since then there has been no successful mass murder at an Israeli school. Peru and the Philippines do the same thing with similar results
I can't see why they don't bulletproof the classroom doors, it would limit the number killed at least?