I'm in 2 minds about this. Are you absolutely sure this is what happened, exactly? I find it hard to believe that a member of staff would keep a whole class back so late that all the other buses had gone. What about the other children in the class - which bus were they getting? I also find it hard to understandt that the teacher didn't ask everyone what bus they actually needed before "putting them all on the same coach." I just can't see how this would happen, unless it's a miscommunciation between staff. I work in schools and you'd ask the child "which bus do you need?" and check with the driver that it was the correct one for all of the children while shoving them onto the bus, (if this was outside the usual going home routine for the school).
If a teacher genuinely kept them late, didn't give phones back, didn't ask children which bus they needed or made sure it was the right bus, then yes, that is totally wrong of them and there needs to be some staff training done pronto (it's not even staff training, is it, let's face it - it's lack of common sense)
However it happened, I fail to see how you keep being in floods of tears about it. Your child is ok. How they get over this little hitch depends on how you approach it. How do you expect to teach your children resilience if you keep crying every time something doesn't quite go to plan? Even though you will be doing that in private, your child will overhear you discussing it or even sense from your body language that you are overwrougth. You need to teach them that even teachers are human and sometimes make mistakes, that EVERYONE does, and because of this that they must NOT be scared to ask for things like their own property (phone) back and speak up if they think it might be the wrong coach (your child may not even have realised it was the wrong one)
I'm speaking from experience. I was a quiet, didn't say boo to a goose, type of child who wouldn't dare to question an adult. But that was the 70s and 80s, and you knew your place and lots of parents instilled into their children never to question adults, especially teachers. I was terrifed of them, and putting a foot out of line by accidentally "answering back". Things are quite different now and children MUST be made to realise that they have a voice and should use it. as it will help them to help themselves.