Why Have I Changed In The Last Few Years ?

See when I was young I was the perfect boy and since I turned 13 I have undergone serious change !! My grades went from minimum 85 out 100 to maximum 60 out of 100 !! From being the most respectful child to the rudest son a parent could have !! I seriousley dont know why is this happening !! And I cant change myself to being good !! I just flunked math (18 out of 100) and my parents have gone mad !! PLEASE GIMME ADVICE !!