You need to get fast.
Unless your usual way of working in class is on a computer, you are going to have to write in your exams. Not only that, you will need to be able to write quickly.
This is a very useful skill to develop, especially if you plan to continue to college.
How can you do this?
Ask your teacher for sample questions. Time yourself when you answer them. Do this regularly.
Find other opportunities to write: keep a diary, write letters to people, write a short story. The more you write, the quicker you will get.
Don't worry too much about being neat, just make sure it is legible. The examiner needs to be able to read your ideas otherwise it is a waste of time.
Join up your writing; some students print. This is a slow way of doing it.
Always plan. It is amazing how many times I will tell a class about the genius of planning and they just ignore it and go straight into writing, then wonder why their work lacks structure and coherence. Planning also helps you to make sure you've picked the best task for you, rather than getting part way through your writing and deciding you've chosen to do the wrong thing.
Planning organises your points and helps you to clear up any mental clutter so that when you start writing, you are entirely focused on the question.
Ultimately, any long answer in English revolves around making key points, whether you are doing a reading or writing task. The points become your outline/structure and then you pad each paragraph with either your quotes and analysis (reading) or your reasons and supporting evidence (non fiction writing). Planning these points takes five minutes at the start of your question that could save you a huge amount of pain as it avoids running out of ideas part way through or repeating yourself.
3. Check your grip and make sure you are using the best pen for you
Many people experience discomfort when writing. I know that I get tense when marking and this can hurt my hand, arm and shoulder. Also, I see lots of students trying to write with an old, chewed up biro or asking to borrow a pen because they don't bother to bring one with them. Using a pen that is right for you can make a huge difference to your writing. There are so many on the market now that have been carefully designed to make writing easier.
Try out different pens and find one that feels comfortable to hold and that you don't need to grip too hard. This is really worth a bit of investment as physical pain can make writing in your exams much harder than it needs to be. Remember to buy black ink.
Try not to press too hard. Joining up helps to make your writing flow better and prevents too much pressure.
Have breaks: stretch and wiggle your fingers. Try rotating your hands at the wrist, both ways.
Once you have picked a brand of pen that suits you, stick with it so you are really used to writing with your pen before you sit your exams. Make sure you buy spares!
Always resist the urge to write in pencil, even if it is more comfortable. You are not allowed to write in pencil for your exams. Get used to using your pen.