I have found a couple of effective techniques in working with teens
(they are also effective with older mothers:)...)

Bring a nursing baby model, one about 6 months old who likes to nurse
and smile and goo and is absolutely adorable, happy etc.  Then
instruct the mother to model discreet nursing first and talk about
what you can't see and how one can nurse in front of others, then
teach them how to tell if baby is getting enough and focus on a
proper latch and swallowing, then have the mom demonstate nursing and
have them watch for swallowing, flanged lips, adequate grasp etc..
Instruct the mother not to make eye contact with the pregnant moms
while nursing (they'll be more comfortable watching), they will be
excited to see swallowing and as the milk lets down they'll be amazed.
 They won't remember anything you say but they will never forget
watching this mother nurse and will want to feel that closeness with
their babies.

I also ask each mom to write a question about BF and hand it in, we
answer those before going on to anything else, there are always some
wild questions about HIV, drugs, sex or alcohol so be prepared- calm ,
thoughtful, respectful answers will build your credibility.

Another technique is to start a discussion on baby care and support
and who will love the baby and how you show love to a baby and how  a
baby shows love to you and how you can make sure to get your share
with all the other people who will love and care about and care for
the baby.  They always tell me more about breastfeeding than I could
ever tell them.

Finally, the most successful technique I have seen working with teens
is a breastfeeding teen peer counselor.  They will have more impact
on these young women than I could ever hope to have.

These simple things have helped in the past, I also have an outline
somewhere which I have used, but now prefer the modeling technique.
Good luck.