Heidi, we had the same problem last week when the kids were taken into the hall and sat down in the places where they will be sitting this week.
The school gave them some maths questions, my dd took one look and froze. Then her hair clip fell out which was just the icing on the cake. She promptly burst into tears and couldn't do a thing.
Up until this point she had been quite laidback. We had tears that evening but I took her around to her tutor's house - we had the last lesson already booked - and the tutor calmed her down.
Apparently this sort of thing is very common. DD was fine for Kent last Sat and came out with a big smile on her face. I can't work out though if it's because she'd felt she'd done well, relief or that she simply didn't care less!