It's a very messy situation that will leave life-long scars no matter which options are taken.
I would think twice about forcing the girl to put the baby up for adoption if she's opposed to it, and here's why- I knew a young girl who got pregnant, and against her will, her family forced her to give up her baby. MUCH wailing and gnashing of teeth on this young girl's part! So you know what this girl did after she was forced to give up her baby? She went out and got pregnant again and ran away from home - saying no one will take that baby from her. Her life has continued to spiral out of control. The last I heard, her life is a wreck - years later. IDK if forcing her to give up her baby against her will triggered all this or not, but it sure seemed to have a very negative impact on her and possibly made matters worse.
I know of another family who went through the same thing. Their young daughter got pregnant. She delivered the baby and the whole family helped her raise the child. The bio dad was virtually out of the picture (a 10th grader) and the girl's family preferred him being out of the picture. It worked out well, but she had a large circle of support from family to help her. It wasn't easy for them at all, but it did work out in the end.