As someone who was bullied ( when your a bisexual , punk rock, actor in a small town, kinda hard to get by. Not to mention that until about grade 11 or so i was pushing 300 pounds. ) as a kid the only advice i can give is advice that people today don't want to hear.
Fight, and fight dirty.
No one is around there friends 24/7, and no one has eyes in the back of their head. Your going to take some beatings, and your going to get into trouble, but after a few goes, and a few heads bounced off urinals, you present a risk, and simply out of a want for a lack of hassle you will be left more alone.
And fighting dirty doesn't mean, bring a knife, it doesn't mean beat someones skull to a pulp. Property damage was the motto of the punk group in my area. Someone harassing you in a car? Well, your a kid, more than likely it is their parents car, and more than likely they picked a spot where no one was around. There are many ways to shatter a windshield that can be carried around with a person. Touch up paint can be used to make a point to someone if you don't want to throw a punch, an open bottle quickly turns a pair of 200 dollar shoes into modern art. Eventually the parents are going to wonder why this keeps happening. And while you don't have power, they certainly do.
There is another option, to simply take the crap. If you don't want to deal with the blowback, and you don't want to deal with the trouble that you will get into, simply get by, high school isn't forever.
Personally, i would suggest a mixture of the two, be unpredictable. What i always did, and i came out of my teenage years with a minimum of scars, emotional or otherwise.
This whole " Defeat hate with love." mantra, is doing nothing more than taking the power the victims have away. Sure, we would all love to live in a world without violence, or retribution, but utopia is for Utopians. And , especially in high school, no one is a Utopian.