I was very sad to hear about a local Chinese student who committed suicide over exam stress. Since I started learning Chinese I've always tried to meet Chinese students, and I always try to help students I know to cope with stress, but there's no way I can meet everybody. I hope some of them will at least read this page.
I'm not a professional psychologist or counsellor, but I have sat Cambridge University's Computer Science examinations, and in my finals they gave me and one other student the joint highest mark of the year. Perhaps that puts me in a position to say this:
Exams are broken!
Well I'd better explain myself (seeing as the University provides my web space and I'm not supposed to say anything that brings them into disrepute)---there was once a British Prime Minister called Winston Churchill who said democracy is a bad system, but he thought all the other systems were worse. In the same way, perhaps the examination system is the best a university could do and all the other systems would be worse, or perhaps we need to wait for future research to prove what sort of system would be better. I'm not qualified to say what's better. But I still think exams are flawed as a way of measuring students. (And the fact that I was joint top first proves I'm not just saying that out of not liking my own mark. But I don't normally mention my marks except when I want to make this point.)
Because the system is flawed, you shouldn't think that the mark they give you is a true measure of your intrinsic value or usefulness. That would be like asking me (a partially-sighted person) to go to a dark countryside village at night and count the stars in the sky. I can sometimes see there are stars, but I miss most of them, and you'd better not ask me about their apparent or absolute magnitudes.
Regarding the effect of one's mark on future job opportunities, here's another thing that might be a little surprising for some: I've been refused jobs and told I'm "overqualified"---and I'm talking about respectable programming jobs here. So it's not necessarily true that higher marks mean more job opportunities. Whatever mark you get (even zero), there will be something (maybe even something that those of us with higher marks won't be trusted to do!) that you can find.
First years might especially need to set their minds at rest: university examinations are not fiery torture chambers. Invigilators don't march around in Star Trek alien costumes chanting "you will be examined---resistance is futile." Universities typically say you must pass the first-year exams to continue to the second year, but in reality the pass mark is not that high because they want students to continue---after all, losing students means losing money.
我特別同情私立學校畢業生。我自己的學校是本地政府所資助的所以我沒經驗文法學校的氣氛，但聽說那個氣氛是“你必須贏，你必須贏，你必須贏”。跟你分享一個秘密: 不是必須的! 你可以選擇試試做考試，也可以隨時選擇離開。如果想起來都是你所選擇的事而不是必須的事，壓力可能小一點。
I'm particularly sympathetic to private-school students, as with my state-school background I didn't have to cope with the allegedly "you must win, you must win, you must win" atmosphere of a grammar school. Let me tell you a secret: you don't have to do anything! You can choose to stay and try the exam, or you can choose to leave at any time---if you remember it's your choice, not something you have to do, then the pressure may seem to be off a bit.
And as none of my my parents or grandparents attended university, I didn't have any family members saying "here's my mark, now you beat it". If you have someone like that, please remember that every year-group is different and the system changes from year to year.
Before my exams I revised every day, but not too much. There was a very good student in my year who revised for 12 hours a day---my 4 to 6 hours weren't going to hold a candle to that, I thought. But he got what seemed to be burnout, and didn't achieve the mark he wanted. (Not to worry: he went on to start a famous Internet company. But it goes to show that you shouldn't exhaust yourself with excessive revision.)
Before the exam, make sure you know the time and place and your candidate number, and if it's in a room you've never seen before then it might be a good idea to visit the room in advance so you know you'll be able to find it easily on the day. When the exam starts, you'll probably see somebody start to write and write and write. Don't let that dishearten you. That student is probably doing it wrong. Perhaps he didn't read the instructions, such as the one about answering only 5 questions, or making sure you've received all 12 pages. Perhaps he didn't read the questions slowly enough to properly understand what kind of answer the examiners are really looking for. Or maybe his handwriting will be too messy to read. You on the other hand can take more care and avoid these traps. And don't use correction fluid (that stuff could be blotting out the right answer); instead leave plenty of space in case you need to make corrections later (this isn't the time to save paper). Remember too that if the exam is particularly hard the marks will likely be moderated upwards. If you enjoy your chosen subject, this might offset the adrenaline somewhat. Don't worry too much about the marks you can't get, but pick up the ones you can. The system may be broken, but we can still try to make the best of a bad job. But whatever happens, never think your true worth depends on some 'imperfect but best we can do' examination system.
Further reading about stress in general (I'm aware the publisher is religious and there are different religious opinions, but it's hard to fault that article)