It's quite a normal situation, to be honest.
Really, you just have to be reassuring. It sounds as if the school have it under control - it's normal to evacuate the class rather than the child in question. Usually the class know exactly what to do, where to go, etc.
If you are concerned that the child is not getting the support he/she needs, then you should definitely discuss this with the HT.
You of course can not go around discussing what you see in the class with other parents, but I would definitely be hugely sympathetic and supportive to the child's parents. They are often ostracised and seen to be at fault, whereas they are fighting for their child to be supported to prevent violence, with little help available. It's an isolating place to be.
So, if you feel the child does not have enough support (ie 1-1 that can recognise and prevent triggers, and help child learn coping mechanisms) please do talk to HT. If your concern is not for the child in question, and for the emotional wellbeing of the other children in the class, then you just need to carry on as usual and be supportive. Kids in these classes manage fine, from infants up, and they fully understand that x cannot control his/ her emotions and struggles. They are very empathetic - often much more so than the parent who wants the kid out at all costs.