1. What empirical evidence is there that arming qualified teachers is more of an accident waiting to happen than having an armed guard? There is none.
2. There is a natural bias against hurting an innocent based on our emotional fast mental processes. However, your rational mind should make this calculation.
Gun man comes into a class and unopposed will kill 20 or 30. The brave teacher will be shot down in a futile charge. Yes, I know some tackles have worked but many don't.
Gun man comes into a class - armed qualified teacher engages. Will 20 or 30 kids be hit by friendly fire. Probably not. Let's say 2 get hit (a tragedy), 28 are saved. The moral onus is on the gun man and not the teacher that tried his or her best. That is especially the case, if the latter took the time to train and made a sincere effort to be prepared.