Thanks for your letter
It sounds as though you are in a serious relationship which you have thought through with your girlfriend.
I understand what you are saying that you are both sure of having a sexual relationship and not hurting anyone.
I would advise though that you need to consider, if you haven't already, of taking precautions and being safe. If you are making sure that you are using protection have a look at our information about sex and contraception for more advice.
What we think is simple or harmless in life may not always be lawful. Yes you are right if you are under the age of 16 it is against the law to engage in sexual activity.
Can I ask why the teacher is starting to ask your girfriend questions? Has something happened?
You should think about what would happen if you and your girlfriend had a row and fell out with one another. Fore example if she accused you of forcing her into sex and her parents found out, or she fell pregnant.
I would advise you both think of the possible outcomes if you started a sexual relationship whilst both young, and I'm assuming, are in full time education.
You could think about talking to your parents or talking to ChildLine if you want to.
Take care and be safe
Sam