problem is that he is not only 12 but he wants sex. I havn't told my mum
because she'll flip! I don't know what to do i get alot of stick at school
because of his age aswell! Should i have sex with him?I mean i do want to
but I'm really scared!
The word that jumps out at me from your question is 'scared.'
Scared shouldn't be an emotion you feel when you are deciding to have sex - anticipatory, excited, happy, committed to your decision...these are the things that will be going through the mind of a person who is making a choice she will not regret.
If you feel a need to ask people outside of you what you should do (such as me or your mum), then you probably are not ready to have sex. If you were genuinely, 100% ready, you wouldn't be facing all these emotions and fears.
I understand you might be getting teased because of his age. I would listen to what they were saying and see if there was any truth in the comments. Maybe your friends are seeing something that you aren't?
And maybe they're just wrong. But 12 and 15 is quite an age gap for that time of life. If both of you feel comfortable with each other and would like to keep dating, that's fine. Adding sex to the equation is bound to only make any confusion MORE confusing. Sex can often add huge pressures to a relationship, and a twelve year old is probably not in a place to be able to handle those pressures.
Depending on where you live, his age might be a problem as well. While you are both below the age of consent - at least here in England - the added problem is that his age is WELL below the age of consent. If his parents found out you were having sex and wanted to make trouble for you, they probably could.
It's best to weight up all your options and think seriously. Sex is a huge committment - getting on birth control, scheduling regular visits to a gyno, buying condoms, being able to deal with the stress of possibly getting pregnant, coping with the changes it will have on your relationship. While sex can be fun, loving, and positive, it's usually only those things when you are in a good emotional space.
If you have any doubts at all, you probably are not ready. A good boyfriend would understand and respect that.
Best of luck.