Maybe they will be motivated by statistics? I give out a brochure at my seminar filled with statistics from HSLDA. You can view the brochure online here:
http://www.hslda.org/docs/study/comp200 ... vement.pdf
It may help to actually give them answers to the questions they have. They may be worried about Prom (there are homeschool dances) socialization (they don't have to give up their friends, and they'll make more and have more time to enjoy their friends.) What about college - homeschoolers go to college and get good scholarships all the time. And they may not think of the benefits, like choosing electives they REALLY want to learn, and studying core subjects in a way that interests them.
It can also help to start with a schedule that is FUN. Schedule some plays, museums, or PE activities each week, so they can see the fun side of it.
I hope that helps,
Blessings,
Lee