Well... I fancy this boy for about 3 mouths now. We speak and we are very good friends. I don't know how to turn a good friendship into a good relationship?

Ask him on a date.

But u see his nice to me after skl but in school his a bit rude(reputation)

Why do you want to date someone who is rude to you, especially someone who is rude just to impress his friends?

You should want to date someone who is nice all the time and will let anyone know that you are friends, because do you want him to act like this when you are his 'girlfriend' just to impress his friends? What if they decide they don't like you? Do you think that he is the type to stand up to them? Or dump you on your behind because they don't like you?

Justwantfair is right, why would you want that type of guy?

How do you know if he's not pretending to be nice to you? Maybe he's actually a mean guy. You don't want two-faced guys.

You can do better.