Can't say for certain of course.However older daughter didn't drop any points at all from practice, scoring in the nineties in all, on the day.When she came out I said," How was it"? She said, "like i thought it would be.I kinda enjoyed it". (strange I know)
Daughter no 2 was scoring almost identical scores in practice, using exactly the same materials. She looked worried going in and I did my best to reassure her.When she came out and I asked the same question, she looked worried still and said " It was REALLY scary".She did drop significantly her scores and scraped in - mercifully.
They both have very different personalities.
So no - not every child does drop in their scores but I think it helps to give them some real exam conditions somehow, (which we didn't), because I did not expect her to be SO affected. If I had done that, I would have realised that we needed to work on exam technique, confidence and accuracy under pressure more than we did.
Its impossible to say for sure the reasons.For example the girls in the hall over ran by 50 minutes so had been cooped up for hours.Who knows maybe that unsettled some.My own daughter had been off all week with sinusitis.It is impossible to tell but the more you can do to reduce the variables by, for example test conditions in practice, the better.Just my opinion.