It's a good thing grenade launchers are easy to aim and can't cause any collateral damage whatsoever.
THEY WERE SENDING BACK THE GRENADE LAUNCHERS!
It says so in the article.
Because even though some of the school-police think they need assault rifles (and honestly, having one or two in a locker just in case is not that crazy even though they are unlikely to need them) they know that they have no real use for grenade launchers.
I am guessing that these launchers were the type that are attached to an assault rifle and they got them with the guns and are now returning them as unneeded.
...But as I said before they are keeping the armoured car and packing it with medical supplies and teddy bears. Just. In. Case.