If it's a real kickboxing place (not cardio-kickboxing), it might be. Ask to get a free lesson. Thai kickboxing, while primarily a sport, is supposed to be effective in real fights.
Tae Kwon Do is garbage for self-defense--it's primarily taught as a sport. Any martial art that is THAT focussed on kicking the solar plexus will earn you an ass-kicking against a person who has any understanding of what they're doing. Generally, stay away from MAs that emphasize kicking above the waist.
Most of the traditional martial arts (various styles of kung fu, karate, etc.) are better for building character than they are for fighting. Sure, a guy who has been studying karate for 10 years will be better in a fight than the average guy, but a decent street-fighter could take him apart in an instant. These martial arts' main benefit in fighting situations is giving the fighter confidence, and a bit of speed and muscular control developed through years of practice. You can develop speed, muscular control, and confidence quicker and more effectively in other ways, without having to worry about belts, katas, and bowing to your sensei.
If you want to learn how to defend yourself fairly quickly, look around on the Internet and in your area for schools that emphasize real-life techniques. These courses are often taught in seminars, or in 6-10 week intervals. A lot of times this involves beating the crap out of somone in heavy padding for practice. If a school makes you wait a couple of years to spar, it's not the right school for you.
A boxing gym will teach you how to throw and take punches (if you get in the ring). Gracie jiu-jitsu and western wrestling should be good grappling techniques. Try joining your wrestling team if you're in high school still.
Since your main problem is that bigger guys are always messing with you, your main solution shouldn't be martial arts. Telling an opponent "I know karate" isn't going to stop him from picking on you; in fact, it'll probably draw a laugh. Bigger guys who mess with smaller guys for fun are too insecure to compete with guys their size, so they talk **** to little guys who they know won't talk back. You mostly need to develop an attitude that you won't take **** from anyone regardless of their size, and project it whenever a big guy starts bullying you around. They don't want a fight, and they'll back down in almost all cases (drunks and druggies excluded). There was a guy on my football team in high school who was about 5'8" and 165 lbs. I remember one day one of our teammates, who was well over 6' and 280 lbs (pretty solid too), started giving him **** in the locker room. The littler guy wanted none of it, and while I can't remember what he said to the big guy, I do remember his face and his eyes immediately after he said it, and I remember the big guy shutting up immediately, losing any modicum of ****iness, and walking away. This didn't involve any verbal threat of physical violence, just the right attitude and a stern "**** off".
It's the few times your brain and your attitude don't keep you out of fights that you need to know martial arts.
