That's a toughie! Personally I'd go with what you're comfortable with, I think that's a good way to approach it. I think I'd just tell the teacher that you can't follow the script because it's not true and you don't want to lie to the kids. I think seeing strong, successful young single mothers is great for kids, and I bet you if any of the girls in the class eventually gets pregnant at a young age she will remember you and feel stronger.
Also, is there a reason you have to talk about it at all? It would probably be easiest to avoid talking about altogether, like to change the subject or answer vaguely, if that's possible.

I don't need to talk about it, but the teacher brought up that the kids might asked me questions. So I was just commenting what I would prefer to say. I do not think they will ask me any questions I can't be vague about.

I'm truly disgusted at what the teacher said to you about this. I've done field work in the past with a science class full of middle school students, so I can understand how you'd want to be polite to her (especially if her feedback to any of your advisors ends up weighing against you), but I would try to be truthful to her, even to the point of explaining what it's like to be a teenage mother. Most of those teachers are naive and heartless when it comes to teen pregnancy. Perhaps explaining to her how you really feel about all that would be beneficial, not only for you, but for other students she'll have in her class who are pregnant. If the conversation is done with tact perhaps she could understand and see how she made rash assumptions.

I also love what momnipotent said about being a great support to other girls in the class who many be secretly pregnant or in the future may be pregnant.

I think if it's something that makes you uncomfortable and like you're either not being true to yourself or that it's something you need to hide or act ashamed about, then don't do it. Try to come up with your own script that allows you to be true and still satisfies the other teachers, even if it's something like "I appreciate the curiosity but wish not to discuss my personal life, if anyone has any other questions..". Just because a student asks a question does not mean you are at all obligated to answer it if it's off topic or about you personally.

Well the kids just had February break so I am guessing the majority of the kids totally forgot the conversation, haha. I am sure it will be fine, but I definitely won't be following the script she suggested if there is any discussion.