Interesting point. We do have a program around here called “Parents as Teachers.” The only person I know to have participated is a fourth grade teacher and her husband. Having P.A.T. available to pregnant teenagers would probably be more effective than head start. Depending on how it is arranged, it could be done at a much lower cost, especially if the school has onsite daycare.
A nearby district used to have a program for young mothers. They attended a separate school which had on-site day care, and also had parenting classes as part of their curriculum.
Supposedly it was very successful, but either the district stopped funding it or it was operating under a grant which ran out. Seemed worthwhile to me - lots of teenaged parents in this area.