You have to work 3 years and then on your first day of your 4th year you get tenure. It's not a matter of not being offered a contract. I think many people look at urban schools as a good starting place because if you can prove that you can be successful in, or "handle" a classroom there, you are obviously doing something right. I will be starting my 9th year in September in my school. I can't tell you how many people told me my first year, "If you can handle this, you can handle anything."
When I graduated college in 2000, everyone was saying how it would be no problem finding a teaching job. I ended up taking this job, an hour away from my house because that wasn't really the case, again, unless you were willing to do the urban thing. The commute stinks, but I love working with the kids I have. When you see how much you are a part of these kids' lives, it is well worth it. I had kids on the last day of school that hung around almost an hour after school got out, were in tears, with a death grip on me and they didn't want to leave. As tough as it may be during the year, those things tell me why I do it and keep going back.