Exactly how, if he holds the reloading weapon in one hand and the new magazine in the other? He MIGHT hold both the magazine and the second weapon at once, but he could not easily use it for defense if he was. (Tried it here in my living room, with no stress or rush. Wasn't easy, and certainly led to moments where I couldn't defend myself easily for a moment or two.)
And, once again, we are talking about a room full of elementary school children. Kill the teacher first and the children are just lambs to be slaughtered.