I assume there would be multiple exits despite only one entryway? If not, blatant fire code violation.
Schools would still have multiple doors anyone can exit from. However, those doors need to be locked at all times and constructed in a way that someone cannot shoot or break the glass to enter. Newly constructed schools must have the technology installed that allows control of all doors on the premises. Older schools have to be done the old fashioned way and manually install automatic locked doors after it closes.
To get into the school you have to go through a single point entry. As soon as you enter you have to go through a security checkpoint. If someone decides they want to try something there is an armed guard in place where they are secured enough to shy away from being shot themselves and can take out the shooter.
My concern is you run into another problem and that is people being ambushed while waiting to get in or let's say a student pulls the fire alarm and everyone exits. They have their weapons somewhere nearby where they can start shooting people as they are waiting outside. Same thing if a student opens a door and puts something in place to keep it cracked open (like that guy in Colorado who shot up the movie theatre did), go get their weapons and then re-enter without having to go through the single point entrance. Fact is if someone wants to kill they will find a way to do it.
For those who do not want metal detectors, etc. schools. Well, we all hate TSA and airport security points but how many hijacked planes have we had since 9-11? Are they perfect, absolutely not. Have they at least deterred bad guys absolutely yes. None of these measures will 100% stop school shootings, but they will at least make it more difficult and cut them down significantly.
The reasons why these kids end up shooting schools is a whole different and extremely complex issue to solve.