Quite often in essays I find a number of small slips in spelling and other forms of error. I think these are important, and when I read an essay I usually go through the first page or two very carefully, from a copy-editing point of view, marking any of these slips that I find. If you find in your essay that I've done this, then this advice is for you.
These slips, though not serious in themselves (a) irritate the examiner, (b) make the essay harder to read and understand, and (c) make anyone who reads your work, now or at any later date in life, think you're stupid. If you do have this problem I would recommend you do something about it. You don't have to (it's your life), but if I were on a course that involved a lot of writing, and my degree depended entirely on this, I'd do something about it. What you should do is check very carefully before you hand a piece of work in, and if you can't get a spelling checker for your word processor, get someone who can spell to check it for you. Discover the kinds of mistake you make. Don't make them again. Try to make your essay perfect.