I'm sorry, mom. I have a friend whose son was in military school (he was a real problem at home and this actually helped him.) He made it all the way to his senior year (like your daughter) and in February, left school and moved to the beach to move in with a group of friends. He started painting houses in the spring. His parents were devastated - less than 4 months before graduation and he split.
He did find out how hard life was. A series of dead-end jobs, roommates taking advantage of him, and no high school degree.
They would not let him come home. He didn't really want to, but they didn't give him the option at all. It was a good thing, really. He was (and still is) a really hard personality. Ten years later he is STILL having to realize that we have to WORK to get to the top, that no one will give us better jobs just because we want them.
He did later get his GED, but no college. Later he wanted my friend to pay to send him to Cordon Bleu chef school (he works as a chef in a chain restaurant), but with his entitled attitude and the way he treats people (and his inherent laziness), she decided that at 26 years old, he needed to be on his own and she would not do this for him. I didn't and don't blame her.
I guess that if I were you, I'd tell her that she can't take anything with her except her clothes, and that she won't have a cell phone or vehicle to drive. I'd tell her that if she will wait until May, and actually graduate (not fail her classes), that you will continue to pay for her cell phone, and that you'll help her set up housekeeping.
Is she using birth control? You really ought to take her to your gyn and get her depo provera shots. If she doesn't have reliable birth control, she may end up begging to bring your future grandchild home to you. To me, that's a lot worse than anything else.
Don't push her for a wedding. If she wants to live with him, let that be up to her. Your goal right now should be for her to finish school and not get pregnant. Tell her that this is all you require of her, and then you'll let her go with open heart, and acceptance if it doesn't work out and she wants to come home (provided that she goes to work or community college.)
I know this isn't what you wanted to hear. You're grasping at straws to try to find something to keep the good kid you know you have. However, she sees stars right now and wants a "dorm" living situation without the studying. You can't make her stay, but you can sweeten the pot to just get through the following months. You might even tell her that as long as she comes home every night and goes to school in the morning, (and passes her tests and turns in homework) that she doesn't have a curfew, provided that she has gone to your doctor for birth control.
It's hard when our kids want to grow up too fast. I'm grateful that my older son wanted to go to college and acts mature and independent (for the most part LOL!) I do support him financially for college, but told both of my kids long ago that if they aren't in college (in my older son's circumstance, not just passing, but doing well enough to keep his scholarship since it's substantial), that they cannot live at home. Either they are in school or they are on their own. I'm a year and a half out with the other one. He's not quite the academic that his brother is, but I think that he will be ready to go when it's his turn.
I think that if you present to her your concessions for her to stay home for just these 4 months and finish high school and get that diploma, telling her that this is what you are offering her, a BIG loosening up, that perhaps she'll at least think about it.
My best to you here.
Dawn