Fired from TA position without cause/notice, what to do next? I am a post-doc in the USA and was a teaching assistant in the same university. During my TA appointment, I attended a one-week course outside the city. Prior to leaving, I got permission from the head TA, e-mailed the course instructors, and got one of my colleagues to cover my class and office hours. 

One day before my return, I got an email from the instructors asking why I was absent from the classes. I apologized and explained everything, but they were furious. A week later, I got an email that I was terminated. No reason was given, and I got no reply to my e-mails. I even called HR and got nowhere, but was told that I would be financially compensated. 

I feel that this was unfair, particularly since another TA had done exactly the same thing and nothing happened to her. I do not think they should get away with this. My research PI wants to get involved since he thinks I was not treated fairly. What should I do next? 

UPDATE: I took permission from the head of TA.. instructors told us in the orientation that she is the one in charge. I had one of my colleague who did the same and was not fired! I got one of my fellow TAs to cover my discussion section in exchange to cover her grading which I did! They treated me unfairly as I told them even to take the salary from me for that week. When I mentioned that another colleague did the same and I feel I am biased against, they did not reply. I asked her and she also took the permission from the head of TAs. I do not want money from them but this might show up on my record in the future and I never had any red flags in my resume and I feel I am discriminated against!
Even my PI in research wanted to contact them because he thinks I am
  not treated fairly. What to do next?


First, you should relax.  Being a TA is not a big deal.  

Second, you should wait.  This is a situation where your PI ought to be responsible for helping you out.  They already said they would try, so let them try.

If your PI does not succeed in helping you, then you might follow the university's official complaint procedure.  Keep records of everything just in case.