How fast can a person turned bad?

It seem like this week is not my week, I am out of luck. After the bad incidents yesterday, I managed to reduce some heat between me and Joon Ting. But I still very unlucky. As promise I became a maid for 2 days, so I was told to refill water bottles. Walked past 4M1 classroom, the pupils are outside as usual.( they are the worse class in the level) They decided to pick on me as I am alone and there is a group of them. Zi Yong hitted me on my shoulder, I was very shock, we were quite close friends in lower secondary and now he became so ........(unspeakable). In that moment I became speechless, he was a very well-behaved boy when I know him. After changing classes, everyone changed, not for the better but worse. Any example, Alex use to be very smart and cheerful, but now still in secondary 3, his result drop from a A to F, no kidding. So this all linked back to my title, how fast can a person turned bad?
About the M1s, go find someone your own size, if you all really want to quarrel with me, you stand no chance at all. Waste my time fighting with a bunch of idiots, dunno why they come school for. I can write a story about you guys maybe on my next posts.
NOTE: Not all M1s are like that, only some gangsters wanna-be.