Take a moment and breathe
Being in an exam room is a high stress environment. All of the work your teen has put in leading up to it depends on the next few hours, and theres usually a huge time pressure.
This pressure cooker environment can play funny tricks on your mind. About an hour after my last exam a couple of weeks ago, I realised that I forgot to include something really, really obvious in an essay. It was a point that I knew really well. There was no reason for me to forget it in the exam.
The exam environment can also make you include irrelevant information. Getting good grades is about quality, not quantity, so its equally important that your teen doesnt waffle on about something thats not relevant. They should ask themselves before they launch into something,