First - these 13 posts exhibit what is extraordinary about the people on TFP. Thoughtful, philisophical and analytical (!) discussion driven by the chance we can help Shesus make a difference for even one of these kids. In my city too. Cool.
I grew up in an inner-city school system just like you mention Shesus. I lived in a neighborhood that the few dads still around were drinking beer watching us walk to school at 8am, yelling from the porches at us. (I was also the lone white kid on the street, so maybe it was just me that got shouted at?
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Are there hidden rules? I don't know that they are hidden, but there are definately a different set of rules they live with than suburban schools. When nobody has money, clothes, or a big house to develop cliques with, the only currecy they have is respect - and it gets traded around a lot. A lot of times the quick way to grab some is through fighting as I am sure you have seen with the grade level your teaching. But you also play with the game God gave you and the smaller kids get pretty good at puffing up the chest for a good show when they need to.
You can't change this. No matter what you say or do, after school they are left in that neighborhood, and that is just reality. What you can do is understand ego is important, especially when the child is a little slower than the rest. I would just reccommend trying to talk to them like adults, especially if you have to discipline them or tell them to do something embarrassing in front of others. When you most want to scold or talk down is when you shouldn't. They pick up on your respect showed quickly and you will develop credibility back, a bit anyway. (I know this is easier said than done.)
Also, if you keep a box of protien bars in your desk it may help the kids who are being especially moody. They may not have eaten for a day or two. Just give it to them on the sly and as a reward for something, anything. Don't ask them if they are hungry, just assume it. They would never admit to friends or you if that is the real problem. The ones complaining about being hungry you don't have to worry about!
Lastly, try to pick one kid each day to pull aside, not in front of the other students, and look him or her in the eye and compliment them on something legitimate. It doesn't have to be school related, but tie it back to them being smart. "Jeffrey, you sure know how to make people laugh in class - did you know that most comedians are very smart people? Oh yeah, you have to learn a lot so you can come up with lots of jokes. Chris Rock reads a lot because he can't tell the same stale joke every day!" It's not bullshitting them, because we are all smart, it's just that nobody's told most of them yet. Who knows, that little bit of confidence in themselves at that age, anything can happen... I graduated from a high school with 370 kids, some of them were the smartest, most personable people I have ever met. Only 6 of us went on to college.
Make a difference and thank you for the work you do and caring enough to post the question.
I would enjoy reading posts about your experiences this year sometime - I am sure it is a year you will never forget!