I missed whatever even started this. So what if a door was unlocked, or even open? It's a school, not a fortress. Why the fuck does a teacher need a lawyer to defend against an accusation about the state of a door?
Why wouldn't you?
First off, from a liability standpoint the school's probably getting sued along with the police department. This is the United States and if a door was supposed to be locked and it wasn't or someone didn't follow the drills that's going to be used against them in a civil case.
Second, public opinion is important because blaming someone for facilitating the deaths of 19 children is dangerous. People get killed over shit like that, if you didn't do it and the police say you did do it, you need a lawyer to clear that shit up asap.
I'm somewhat reminded of that Swiss(I think) Air Traffic Controller that was on duty when two planes collided like 20 or 30 years ago. It was on that dude's watch, there's obviously some culpability, but, his employer was doing maintenance on one of the systems that gives heads up when two aircraft are on a collision course and the phone lines were jacked up and he was alone at the time and even though he caught the error and gave orders to one aircraft to correct the issue the 2nd aircraft followed their TCAS directions which led them straight into the other aircraft. It's totally possible another controller would have caught the two aircraft's intersection faster and issued guidance that would have separated them far enough apart that the tragedy never happened, the employee has *some* responsibility, but he also shouldn't have been hamstrung with downed equipment either. Regardless, one of the family members of someone killed in that collision caught up with him and killed him.
Obviously, the situation's not exactly the same here but if the official story given to the public is that you saw a gunman, panicked and left a door propped open and gave the gunman access to the school to save yourself, leading to the deaths of 21 people and I don't know how many wounded, it's not a total stretch to imagine some parent not in the right frame of mind holding you responsible for their child's death. Hell, even the police have requested security following the shooting so the police know exactly what can happen when someone's publicly believed to be responsible for the deaths of multiple children.
If the police are saying that you did that and you didn't, you sure as fuck better get a lawyer and force the public story to change, fast.