Typical gut overreaction.
First of all, as many commenters pointed out, the gunman blasted his way in; having been in a Yeshiva where there is a security guard, I can tell you there’s no way he would’ve been stopped (especially considering the intelligence and physical state of many of the security guards that are out there).
Secondly, he was a teacher in the school’s son. Even had he been asked to shown ID, would he not have been allowed in?
Sorry, but Sheep w/o Spleen is right – better to use the money to bring the salaries of the Rebbeim and teachers somewhat in line with what they deserve than to blow it on useless and pointless “security” that exists only to make gullible people feel good.