Why do we dread writing so much? For most students, thinking about writing for an assignment gives wartime-level flashbacks of mental strain, sleep deprivation, and undue stress to meet a deadline. These factors combine all too well with a lack of motivation for writing really anything. Sometimes, though, a student's want for not writing a single word comes from a preconceived notion that they aren't that good at it.
Well, friends, take heart! Writing well is a skill that, like most other acquired skills we pick up over the years, takes practice. While it is true that some people are inherently more adept at some skills than others, that does not mean it's a skill that is doomed to a life of mediocrity. Writing is a skill that must be practiced often, and corrected often, so that it can be improved. It takes plenty of practice to get better at writing in one particular style; it takes even more practice to "master" many styles of writing.
That doesn't really help with your motivation factor though, does it? Sure, you might be able to write better, but it doesn't do you much good to have practiced all of that time if you don't want to use it. Hell, you probably didn't even get up the motivation to work on it all, did you? Every person has different motivations to accomplish their goals; you have to find what could motivate you to be successful. Maybe you're seeking the world of writing riches. Hint: find another one if you aren't fully dedicated. Maybe you want to zip through all of the writing assignments you've got going on at school. Maybe you're just following it as a passion because you enjoy it- we are a rare breed, but we do exist.
I write to relieve stress. It's an oxymoron for most, but I enjoy writing because it takes me to some other place. It's the exact same idea for people who read to relieve stress, but, instead of recanting the story, I love to weave it. It's my motivation, and, of the two things you need to improve your writing, it is most important. It can be as simple as just wanting to write the best possible stuff for class or as complex as wanting to create the next big novel series involving romance and werewolves. (Please don't; the idea has been beaten to death.)
Whatever it takes, find your motivation. Then, practice 'til your fingers fall off. To be perfectly honest, you can attach this advice to pretty much anything so, hey. If you still hate writing more than any other possible thing (except maybe math), apply this to something else that does matter to you. Live your life motivated and free, my friend.