OP, I had this discussion the other day with a friend and we came to an agreement that if possible, we would want a PCC or SBR folded up in a legit backpack. When in between classes, we would walk up and down the halls to generally monitor the school with our inconspicuous backpacks on. Just like I don't want to be caught with a knife at a pistol fight, I also don't want to be caught with a pistol at a rifle fight.

If I HAD to only carry a pistol it would be an RMR'd Glock 17 or Beretta 92 at 5:30 with a quality tuckable IWB holster, button down, tie, jacket and pants. Spare magazines (minimum 2) in either a belly band or similar IWB mag holder.