Patience is the key.....
~i guess u shud spend more time with kid
~ harmonal changes in kids occure usually at the age of 11 and 12, so mood swings are common
~listening him/her what he/she wants
~taking care of the fact who are hislher freinds
~check more often what he.she is watching on TV and internet
~dont confront hi/her, infront of everyone, talk to him alone abt the things bothering you
~before he/she sleeps go and spend time talking abt his/her day and tell abt yours, and this is the time wen i tell my daughter what were the things in all day she has not done right and she needs to improve

i hope i helped,it works with my daughter who is 12 ...and it always works

best of luck