Disregard gurlygurl's answer. Your parent is your parent, not a a sexual predator who you need to deflect by firmly saying "I don't feel comfortable talking about this with you."
Your mother wants to talk to you about this for a very good reason: If she is a good mother, it is because she loves you and wants to protect you.
Sure, you say you have remained abstinent and think you know all there is to know about sex. Which is great - congratulations on keeping yourself knowledgeable on the subject. That said, your mother has no idea of knowing what all it is that you know - and, it may surprise you, but you may not know everything that it is she would like to talk to you about. It could be important and you will probably appreciate it greatly, even if it makes you awkward to talk about it to begin with. Your school is your school, and the teacher has a job to teach children certain things within certain guidelines - Trust me on this, learning it at school and listening to what your parent has to say to you are two totally different things. If a teacher sat down and had a full-blown, one-on-one sex talk with a kid as a parent should, they would probably be fired.
Know that talking to your parent about this is something almost everyone has had to go through, and almost all of them went through it red-faced and awkward, but much more appreciative and knowledgeable in the end from the things their parent has told them. In the end, you ought to feel a lot better that if you have a question in the future, whether you have begun having sex or not, you can go to the people that care about you the most - your parents. What would you rather the alternative be - Go to your 5th grade teacher every time you have a personal, sexual question to discuss? Again, they'd likely be fired for this, and you would also likely prefer discussing it with a trusted parent. Your mother cares about you, which is why she is asking in the first place, and is most likely going to keep trying to start the conversation with you - would you prefer your mother act like she not care about your knowledge on the topic of sex, your body in general, or what you are doing sexually? It may be awkward, but try and willingly have this conversation with her, like an adult. Good luck
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