That's a very good question and one I'm sure all teachers face at one time or another. I think I'd start by saying that if you were good enough, grade-wise to take the test, you must really love your subject. That's a great starting point. Even thugs and gang members can get excited about learning with a teacher who loves sharing his/her knowledge in a way that is interesting to kids.
Secondly, I'm going into teaching with the belief that for every thug in my future classroom, there are at least 5 other students who want to learn so that they can make at least a minimal success of their life in the future. They may not want to be in Spanish (my subject) but even if they never learn the language from me completely, I'll at least have the opportunity to expose them to other cultures.
In today's society, you are going to have thugs, etc in every classroom. Unless you land a job in an exclusive private school that probably doesn't pay as well, you have to accept that fact. But if you really want to teach kids your passion about your subject, you are going to get nowhere fast unless you go into the job loving the idea of sharing your knowledge.
I should of course tell you that I'm not a teacher yet so what the heck do I know? But I am a parent and past the age of 40. I've worked with teenage kids a lot, which led me back to college in the first place. If you search for the good in a kid or something that interests them personally, you are going to be able to teach that kid something. And at the very least, even the thugs will show you respect for trying to get past their "tough" veneer to see the real kid inside.
Good luck. I hope you decide to teach. One of my methods teachers has promised me that someday, when I least expect it, I will hear from or about one of my past students that they picked their career based on what a great teacher I was to them. I hope I live long enough to hear it personally!