Tess - first of all I'm sorry you and your DSD are going through this.
I wanted to say that I don't think an amnio is the only option for a Downs test at this point. It is the only test that would give a definitive yes/no answer but your DSD could also have a blood test (triple test) to give her a risk. So she'd get a figure like 1 in 5000 or 1 in 12000. She can then decide if she thinks the risk is low enough for her not to have the amnio or vice versa. But I think this blood test can only be done up to something like 16+3. So it will need doing asap.
Also I've been on a teenage pregnancy course today as I'm about to start working with pregnant teenagers. One thing that was mentioned is that there is something called a "care to learn" grant that will pay childcare costs so that the teenager can go back to school.
I'm going to be running something called a Young Expectant Parents' course - its equivilent to a GCSE but subject matter is pregnancy, labour and newborn care. Ask if there is something like this in your area.
Also this link is to a teenage pregnancy forum, your DSD may find some support/friendship here from others in a similar situation. She's likely to become isolated from her current peers and this could help.
www.babyandbump.com/teen-pregnancy/
If there's anything specific you want to know then PM me and if I can help I will. I'm back on the course tomorrow and could always ask one of the trainers there stuff, they've been working with pregnant teenagers for years and have loads of experience.