Do you have bad anxiety? Serious question.
Most schools do not have security armed with guns, if any security at all. Many times security won't even have a weapon. They will be there to walk the halls and deal with kids skipping class. So say the school now has metal detectors. The kid walks in, the detector goes off, security walks over, kid blasts him. Then goes about his original plan. Metal detectors won't do stuff.
Now, lets say on the off chance security has guns. You said your school building is huge. The kid is going to know about security and make entry another way. Maybe force a door or smash a window. How would that be stopped. Putting armed security and metal detectors at every door? Multi-bolted doors? Bulletproof glass and bars on all the windows? Hell why not just have class in a prison.
And this is one school out of over 100,000. It just isn't practical. It sucks but if it is going to happen, its going to happen.