Our eighth graders did this at one of my old schools (although we used a program called "CryBabies," which I have since learned was sponsored by a group that also ran pregnancy crisis centers and which was part of California's absurd abstinence-only curriculum, although I think ours was a revamped version).
Aside from the issues above, my concern with any of these programs is that they may have the opposite effect from that which is intended. From what I saw, students received a lot of positive attention during the days when they had the dolls. Rather than, "I am too young to handle having a baby," the take-home message for the youth seemed to be, "Wow, I get a lot of attention when I carry this cute baby around with me. This is cool." While I haven't read any research to support this, it doesn't seem like a huge leap to me to think that, for all of the students who will leave that experience more committed to not having a baby, there will be some students who will come away believing that having a baby will bring them the love and attention they seek. (Heck, the only thing I remember about my own junior high Home and Careers course is getting to carry the egg around and pretend it was a baby! It certainly taught me nothing about caring for children, although I did learn how to make the little My Little Pony carriage I had for it look supercute).
I would be very curious to see some longterm statistics for these programs: I recall reading that the quant. analysis suggested they weren't effective, but I wonder if, more than being ineffective, they actually do more harm than good. To me, this seems far more damaging than young people's knowing that teen moms like Jamie Lynn Spears exist.