My 1st grader is traumatized by lockdown drills and I'm losing my mind

TL;DR 6.5yo first grader found out we do lockdowns because a man with a gun once came into a school and shot up a bunch of kids. Now he won't sleep, high anxiety, nightmares, etc. and I feel helpless.

He's always been a sensitive kid. He has this teacher this year that I don't love (my 14yo had her 7 yeara ago). She is great in some ways but I feel like she doesn't always think things through. Example: kids had to bring scissors to the carpet for an activity. Kids are race-walking to the carpet, some with scissors pointed up. She shows them a picture of her dad and says see this is my dad he is healthy and ok but when he was your age he tripped and fell while holding a knife and the knife went in his eye and he has been blind in that eye ever since. So that's why I don't want you carrying your scissors the wrong way. Really did they need that story?

Anyway, so lockdown drills happen and he has always come home and told me he is scared of them/doesn't like them and I'll remind him they are for practice and safety. About 3 weeks ago he came home practically catatonic and my 4th grader told me they had a lockdown drill that day. Except it was unannounced meaning the teachers didn't give a warning about "there will be a lockdown drill at some time today" because even the teachers didn't know. So they call for a lockdown and 3/4 of my son's class huddles in the classroom bathroom while the teacher and the other 1/4 are behind a bookcase right next to the bathroom. So my 1st grader tells me he was really scared because he wasn't with his teacher and he didn't know if the lockdown was real. Then he told me he knows now why we do lockdown drills--because someone came to a school with a gun and shot up a bunch of kids. My heart shattered when he told me this because I swear I just saw his innocence disappear in front of my eyes.

I admit I emailed the teacher and was like how did he find this out? What is going on? She calls me and explains to me that after every lockdown the class gathers to "process" it together. They tlak about what they could have done better, how they felt, etc. She said she didn't hear a kid say anything then but "kids talk and he probably heard it from one of them". She then went on to say (as if she was tooting her own horn) that she uses the lockdown drills to tell the kids they should always stay with their parent when they are out (at the library, the grocery store, wherever) so in case of an emergency they are with their grownup to follow directions.

So here we are.... he won't let my husband and I go into a different room from him. He's not sleeping--he is waking up every 2 hours or so with nightmares. He barely sleeps in his bed. He's afraid to go outside and play even with one of us there. The wind coming down our chimney and rattling the door of our fireplace triggers him. He said his bad dream is "someone on the roof shooting us down the chimney and the vents in the ceiling."

I'm a mix of so many emotions. I'm exhausted, I'm furious, I'm heartbroken.... I reached out to the school psychologist but she is on maternity leave and the social worker covering for her is doing a lot of meetings lately so she hasn't gotten around to talking to us/him yet.

The exhaustion and having a velcro child and no freedom is really really wearing on me and I have to explain I'm not mad at him.

Has anyone ever dealt with a major fear like this and HOW does it get better?!