I changed my reputation from the annoying creepy kid when I first started high school. It's hard to totally change your reputation, especially if you screwed up badly at first. It'll take time, probably years.
After the summer holidays (or any other school holidays), you get a bit of a clean start. Dumb things you did aren't in the front of everybodys minds.
To help you best, you need to say why people hate you so much.
I was bullied as well, so I started standing up for myself, and ended up being known as quite a badass. I figured it was better to be a jerk than it was to be a nice guy or a weirdo, and I was right. I also decided to stand out. Unfortunately, I did that by being weird, so things still didn't improve, they merely changed. Remember, there is such a thing as bad publicity. Remember that.
Over time, although my bad reputation was fading, it was still there, enough to mean that any chance of success with the girls in my school was permanently dashed. Field trips and excursions and any trip where you meet girls from other school are very good. Your reputation isn't tarnished with them. Being seen with them by people from your school will help to fix your reputation within your school.
I only really stopped doing stupid things socially at my school until my last year. Normally, one or two things can be forgiven, but if you have a bad rep, it'll just remind people of what you have done in the past. You need to stop making the mistakes you have been.
Work out. Eat a lot of protein, get 8 hours of sleep a night, and lift heavy. Stick to it. It'll get the girls after you and the guys respecting you. It's extremely potent in high school.
Learn how to act with women (read sosuave), and don't do anything stupid. Dress better than everyone else and don't bit a victim of the latest trend.
Accentuate your best attributes. Befriend the guys, don't just concentrate on girls, that's very important in high school (not that it's not important in the outside world.) The more friends you have, the more parties you'll be invited to you, and the more people you will meet.
Finally: I can answer your question better if you say exactly why you're not liked.