Hi Hun

Firstly, I have to commend you on being so brave for standing up for your baby, it really takes guts to stand up and take responsibilitiy like this. And with regard to your question, the easiest way to say no is to just come out and say it... Your mom is scared and concerned about you and as F@cked up as it seems... it's her way of trying to protect you from what she sees as making a mistake.
Believe it or not parents need reassurance too, so sit down with her, hahve your bf there (and maybe another adult who you trust, just to help your mom keep her cool) and try to put her mind at ease.
Make a plan wiht your bf too, show her that you guys are serious about this and that you know what you're letting yourselves in for. Reassure her that you will finish school and that this is not the end of your life... it's the start of a new one.
If she's still stubborn, get her to read testimonials or blogs by girls who have had abortions and are still suffering the after affects of it. It'l really wake her up.
I wish you all the luck in the world with your mom and for a healthy pregnancy.
Love-Eva