You could also ask someone independant to talk to her - a teacher or support worker at school, a medical professional (she will need a GP check up and they may refer here somewhere anyway), or a midwife, social services or something - all will be involved later on anyway.
How about finding some young single mums for her to talk to, so they can tell her about their experiences? Any of the above organisations could probably help with this.
Has she thought about how she would support the baby? Where they would live etc? I know you would help her, but is she just making an assumption that you will without realising the effects it could have on family income and daily live?
What are you thinking are the alternatives to her having the baby? If you're thinking of termination, then, to me at least, that is a much harder thing to live with than actually having the baby - and not always the best option. If she does chose this, there may be repurcussions down the line that will be difficult for everyone.
If she's only just confessed to you, I would leave her calm down a bit, then engineer an opportunity to have a conversation somewhere where she can't get away - on a car journey perhaps.