Daughter seperation anxiety; I spoke too soon

Spoke too soon about having the seperation anxiety almost beat... Note to self: Never be late!
So in my first post about seperation anxiety with my daughter I was saying how she was doing so well. Guess what? She went to band and I was on my way to get her. A train goes through our town and I had to stop for it. I was starting to panic thinking oh shit shes going to panic. Then I thought, well I was ten minutes early the last time and they didn't get done until dxactly 3:30 so its 3:29 I should be good. I walk in the door and her face was beet red, she was crying, standing by the phone and the teacher says she thought that you were going to forget her? What? Made me sound like I just leave her everywhere.
I told the music teacher that she has an anxiety problem, she said yea she told me and looked at me like I was a nut.
Got into the car and I told her I was on time it ended at 3:30. She said she knew but was still crying. I let her settle down and tried to change the subject by making her look at the ducks, trees, anything.
When she settled down I asked her if she took a breath when I wasn't there. She said yea and it didn't help. So I had to explain that she may need to take more than one breath. lol its really not funny. but made me smile cause I'm so stupid. So I went through the whole bit but kinda forgot how to handle it. It has been a long time since she had done this. So I told her why I was on time, but late. I explained that this is life and stuff like this is going to happen from time to time. Wrong move. Her anxiety went through the roof. She said "When? Are you going to be late next week? Are you sure?" Her voice gets high then low and she talks really fast.
 Had to start again.
Asked her what she could have done when she started getting nervous, call my cell phone. She said she tried but the school phone wouldn't work. Well how would she know that you have to dial 9 to get out and be able to call. Now she knows but the teacher was right by her you would think if a kid is sitting there crying and freaking out they could help them call out. Or she may have told her but my daughter doesn't hear anything seriously when she is like this. So now she knows that. Although I hope to God she doesnt need it.
So she tried the phone thing and it didn't work.
When I walked in the kids were looking at her like she was nuts too. Kinda sad. They were asking her what was wrong but when she tells them they just don't understand. Most of them were waiting too or just then calling their parents.
So went on. I asked her if maybe she could have walked outside the door and looked for my car because its hard to find a space, oh no! What if she couldn't get back into the room! She goes to band at a different school too so its not like she really knows anyone and we are pretty new in this town so none of the parents would know her.
After 4 hours I finally had to get a little tough. But again don't do this if they are really bad and haven't had the chance to learn how to deal with all of this. I asked her what the worse thing that could have happened, which wouldn't because I have never forgotten her and if something happened I have four people who she knows would be there to get her.  But I finally did tell her "Ok so if nobody showed up what would you have to do? Or what could you do?" I could see her face turn red and I told her to breathe. I asked her if she could have made it home? Yes.
Could she have waited in the school until someone got ahold of me or someone in her family? Yes. Can they  just leave you in the school and leave? No I told her they cant do that. Worst things that could happen seem horrible to kids with seperation anxiety. I try not to do it but sometimes have to and she has band today. By God I will be there, early.
Note: I will be going to other school to music teacher and tell her about my little one. It is horrible for these kids and she feels better if the teacher knows. Thirty seconds is a lifetime for them, its sad and almost unbelievable that a child can get so anxious that fast but they do. My mistake thinking she was well enough that the other teacher didn't need to be informed about her anxiety.
Small little things like this takes these kids back. She bounced back pretty fast from this one. It used to take a week to get her over one incident. So shes better but still delicate and I can't forget this.