It can get very sore. I remember the jaw and headaches I'd get right after the appointment, but I'm not so sure about the initial fitting.
The orthodontist should tell you whether he's put any tension on them, and whether your dd should expect some discomfort. Soft food and a couple of paracetamol always did the trick for me and I was fine at school
Our ortho tells us to give DS paracetamol straight after the appointment before a headache sets in. He has top and bottom braces and has them adjusted every 6 weeks and usually goes straight back to school afterwards.
Are these her first braces, or is she used to wearing some form of braces anyway?
ds had block braces first and then moved on to fixed braces, so he was used to wearing braces and so the eating food side of things was actually easier with the fixed braces.
It is extremely likely that she will have an achy jaw / headache for a while afterwards, so make sure you have paracetamol to hand. This is true after every fitting, as they change the wires to make the braces gradually tighter.
It's your call, but I think if it was me, I would send her to school with paracetamol. She can always let her teacher know she's not well and come home again if necessary.