Well, once the teen is pregnant, there is no GOOD answer. As difficult as it would be, I would encourage her towards adoption. Why? It's the least of the evils.
First, as a Christian, I cannot justify the idea of abortion for moral reasons. Even if you look beyond that, I knew several people in college who had abortions, and each of them had a hard time "moving on" once it was done -- one even tried to commit suicide, dropped out of school, and was generally a mess for about a year. As an adult, I know several women who still grieve over children to whom they never gave birth; they occasionally mention how old their children would've been by now and wonder if they'd have been girls or boys. It's entirely possible that some of my other friends had trouble-free abortions, and I never knew of it; however, in my experience, there's the possibility of tremendous emotional trauma.
For a young teen, keeping a baby means the end of her own childhood. The rest of her education would be more difficult, in her early career years she'd be burdened by caring for a small child, and as a teen she'd have very little to offer a child.
Going through a pregnancy and delivery would be difficult for a teen, and giving up the baby would be difficult -- I won't argue that for a moment. But at least once it was finished, she could have her life back. She could continue with school and focus on her own future. It wouldn't be problem-free, but once an unwanted teen pregnancy has occured, NO CHOICE is going to be problem-free.
I'm sure someone will say, "You'd give away your grandchild???" Again, there's not a good choice. But, yes, I would choose my child, whom I have known, nurtured, and loved for more than a decade over an unplanned baby to whom none of us have an emotional connection yet. I would choose to let her go on with her life with the least amount of disruption.
The real answer, though, is to PREVENT the pregnancy in the first place.