Re: Columbine, my understanding is that there was one LEO assigned ot the school. At the time of the attack, he was on his lunch break, parked across the street at a park in his vehicle (I don't know whether or not the vehicle was marked, or clearly visible to the shooters. Because it was the norm for the school to have a LEO on premises, the two shooters likely knew that, and may have timed their attack to coincide with his break.)
Again, according to my understanding, he did indeed respond to the attack, trading a few shots with one of the shooters, who then turned and re-entered the building. As was police procedure at that time, the deputy then held his position outside and called for backup and a SWAT response.
Those procedures, as most of us now know, have been changed to have the first-responding LEOs now "charge forth and engage", presumably in the hopes (backed by statistical outcomes) of either neutralizing the attacker, or inducing him to commit suicide.