Personally I feel like a lot of people are being particularly harsh towards the OP. It's very easy to sit in an exam, be worried about it and totally forget very trivial things (for example, yesterday I had an exam I was really nervous about, and I forgot to switch my phone off. Luckily it was on silent in my bag.)
The point is, it's an exam and it's a stressful time. If this was the second time the OP has done this then I'd laugh too, but it's a novice mistake and I'm sure you've all done it, you just did it in nursery/primary/some time pre GCSE when it didn't matter.