Flashbacks can be a symptom of PTSD but true flashbacks happen while awake and they go on for months after the event that triggered them. I think what you are experiencing sounds like a normal reaction to a really diificult situation which should resolve in the next few days or may take a little longer. I think that while being reassured that there was nothing you could do you might feel better if you do do something. Was this one student who did 2 dangerous things or incidents involving 2 different students? If it was the former I would be trying to get some additional support or this student and trying to find out what was going on with them If it was the latter you probably need to do afull risk assessment of the classroom, materials and equipment being used etc . This should help you be clear in your own mind that everything you could do to keep students safe has been done.