Gun Control Advocates Misinformation by Guard in Columbine Attack
1) The guard at the Columbine High School attack did delay and that allowed many students to escape out of the building.
2) The guard was only able to delay the killers for a while because they had homemade grenades. The guard was hiding around a corner in a hallway, but when the two killers started lobbing their grenades down the hallway his position became untenable. Despite the claims to the contrary, it wasn't the "assault weapons" that were important in making it so that the guard had to back down. (Some information is available here.)
3) Finally, the officer at Columbine was there because he was such a bad shot. He was not given regular duty and was assigned to the school because it was deemed that his ability to properly shoot his gun wasn't thought to be an issue.