I didn't do sciences, but honestly, just sit every paper like it's finals. Study all the content you've learned long and hard, time yourself during papers, study ahead of class and by the time finals come, you'll be well versed enough to not be stressed. Make folders for each new section of the syllabus you cover and go over them weekly, at least. Start doing practice papers on your own early next year and definitely don't waste any free periods this year. Join discussion groups too, you never know how much you didn't understand something until you have to explain it and correct others.