Grade my class's behaviour and attitude

Sometime, I do not really know how to grade my class's behaviours, which I found that it is quite ridiculous, out of what I expected.

My class has 2 "gangsters", whereby they pierced their ears, lips or tongue, and dyed their hair "colourful-ly", "blowing" cigarettes, but their behaviour really is not what I expected at first. I thought that they will always make fun of people, teasing people, trying to turn the class upside down, but I was totally wrong. Yes, they do not pay attention to lessons and often report late to school, but they do not really go round and disturb people. Instead, sometimes, they will even tell me "becareful" in a polite manner, when they are moving chairs and tables around. They would also greet me, by saying a simple "hello", or addressing my name. Also, when they are requesting for something from me, they used polite tone in conversation. Their attitude and behaviour really "shocked" me "positively".

But, what really makes me felt "overturned" was my "bunch" of chinese friends! I thought their attitude or behaviours would be quite alright. Sad to say, they actually "do not see well" for people who take Food and Nutrition! How i wish, I could give them each a tight slap for this comment! What's more, they even used that as a slogan when we did our work on advertisement! It's totally ridiculous, arent they?

Overall, their behaviour really "shocked" me deeply. Hope that the bad ones will change and the good ones will continue to keep it up.