From what I heard (If this is the shooting in Texas...kinda sad I got verify it) the kid apparently came in with a shot gun and they later found out he put explosives around the school on and off the perimeter of the school ground. It's really sad honestly.
Honestly, I feel it is not the guns that need banned. It's school security and security in other areas. You can tell with all of these school shootings and shootings in general. They are all planned out. They are all things that took time to think of. If someone SERIOUSLY and truly wishes to kill a crap ton of people in a school. Guns or not they will probably find a way to do it. There is literally things like bombs which are extremely easy to make I've made some fairly powerful ones myself using nothing but stuff in my kitchen for fun. There is still also things like poison fumes that don't give off much of a smell and dozens upon dozens of things people could do.
Personally I feel we should be more targeted on improving school security and also doing things like stopping guns from getting into the hands of people and ensuring peoples safety.