Dumb but friendly or smart but cold?

There once was a little girl called Riho who used to talk to me throughout the lesson (mainly about Prison Break), show me her new pencil case and laugh at all of my cheesy jokes.  She was very good at English and I even managed to piss off her older sister by saying "Riho is much better at English than you were at her age!"  There now is a seventeen year old Riho who is in the upper stream, hangs out with all the popular and smart girls and is, well, rather indifferent to me!  I didn't teach her class at all last year, so was looking forward to getting them back this year.  They are very obedient and do all the work with the minimum of iyaaaaa (FFS! God! Sigh! Tut etc that you'd expect teenagers to utter).  They are a tight unit and recently they have made it clear that there is a wall between me and them that I have no right to try and climb over.  I was quite depressed in today's writing class: they cottoned on straightaway, I got through all that I wanted to and they readily accepted that they should finish for homework, but they didn't ask me one question. There was no banter: there were a few jokes between themselves, but I wasn't invited in. 

Compare this with the third year high school class.  They are supposed to be the top class of the senior year, but half of them don't belong there, some of them are struggling with vocabulary and grammar that they should have mastered in junior high school.  I moved six chatterboxes to the front of the class. I despaired that only 50% handed in their homework (which was already two weeks late!) Half the class struggled with the same material that Riho's class (a year younger) did with ease.  They said that it was impossible to complete their homework by tomorrow - "I know you said write 100 words, but how about 50?" Yuri continued to disrupt her classmates even after I had moved her.  I told them "Kuro-chan (a comedian Yuri is in love with) only likes girls who do their homework and are not annoying!"  She said "I don't think so!" I said "It's true, I spoke to him last night and he told me that he only dates girls who write 100 words."  They said "Rebecca, you are so full of shit!"   I had a bit of a sore throat at the end of the class, but I guess it was kind of fun.