The columbine bombs were, iirc, incapable of exploding off of the timer/detonator that they had put in place. A tank full of propane contains no oxygen and hence cannot blow up. the only thing that it can do is to be released into an oxygen rich environment. If a valve is popped or a hole punched in it and the gas catches fire, there will be nothing but a large torch, depending on how much LP is escaping. Propane will only explode when it is gasified and held in an oxygen rich environment at a concentration that is high enough to sustain combustion.The other way of doing it would be to set the thing to burst somehow, flood the area with propane gas, and have a timed spark emitter. Set it up with a bunch of lighted candles concealed somewhere and an electronic device that will open the valve? I can think of several ways to do so.
IIRC, only one explosive device actually worked, probably because they were stupid. They duct taped nails to the outside of the tanks, right? There won't be an explosion that could turn those nails into flying shrapnel.
Sometimes people engage in 'magical thinking.' This refers to thoughts and actions that really aren't realistic, unproven, so forth. "propane explodes, tanks can explode, if we wire a timer to a tank and wrap it with nails, something will happen and it's going to kill everyone in the cafeteria."
In fact, it appears to be impossible or at least
nearly impossible to even create an explosion with a tank and a low energy source Tracer bullets from 'normal' calibers blow out. There isn't a fuel air mix that can support the combustion of the gas until the tracer and its heat are gone.. LP is cold, wet, and prone to creating quite a wind. It isn't that simple to pop it off.
There was a comic that I once saw that has stuck with me for decades.
Sidney harris, found at blogspot, contemplative day book
before anyone refers to the USAF fuel/air bombs, remember that they include
air. Until that fuel reaches the proper mixture it can't ignite. There are two ways that i know of that this is addressed. First, those bombs carry a large bursting charge that sprays LP in a wide sphere, and gives it a few seconds or so to mix with the atmosphere before it is triggered by a hot, large explosive device. Another way is to add an oxydizer directly to the fuel as it is dispersed, so that the explosive isn't entirely dependent on atmosphere.