If you speak to the school they can usually test you if they believe you really are having difficulties, and then if the test was positive you'd go from there about the exam times. Speak to the head of year or the 'special needs co-ordinator' or similar, or if you don't know just ask a teacher who you like and they can point you in the right direction. And if you have real difficulty finishing exams in time consistently and your teachers deem it's holding back your potential I've been told you can sometimes qualify for a little extra time... but only in rare cases. You'd need to speak to exams office about that, and really try at just speeding up first. I used to never finish exams and underachieve because of it, but I just had to work at speeding up.
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