I know they could put up 12 foot high solid concrete walls, have checkpoints where everyone is strip searched upon entry whilst being covered by a sniper to make sure they don't try anything (and then have another sniper in case the first gets funky, who is covered by a third), and then have everyone in individual cubicles that they're only let out of when there are at least two armed guards per class (you need to have them able to cover each other) and an armed teacher to escort them to the next class.
You might want to make them all "boarding" schools to reduce the time wasted checking the children in and out.
Which is actually the logical conclusion to what the NRA and their political wing the Republican party are basically suggesting (especially given the level of security and how the kids are treated in many US schools by their current security).