Added: Not sure how you actually are able to answer your own question (a So What Happened is normal practice), but in regards to skipping school:
A couple of moms here have said that the way to deal with that is to go to school with your kid. Walk him from class to class. As soon as school is over, walk him to the tutor, then pick him up and take him home. He should be absolutely humiliated that you are doing this. Of course, if you work outside the home, you can't spend the days at school. But it has put the "kabosh" on the other kids skipping. If the girl's mother is a SAHM, she should do the same.
After school, whichever parent is there, let them see each other and sit down together and do their homework and study. In the kitchen or dining room. Treat her like she is part of your family without being mean, but expecting her to go by your rules. Tell them that they are not allowed in the bedroom - only in the gameroom. Tell the brothers not to "disappear". Tell both your son and his gf that if they don't follow the rules in your home, you will drive her home and she can't come over for 3 days. Tell them both that they are an example in your house to the other kids and they need to remember that as older teens, they set the bar. And you are not interested in being a grandmother this early, and you are sure that her mother is not interested in being a grandmother this early either.
Your son sounds like he is not interested in college. Time to get him interested in a career. Tell him that if he is going to end up getting married, he will have to be able to provide for them. Tell him that they cannot live with you. Go show him apartments, let him look at a lease agreement, ask the apartment manager to run the numbers. Take him to look at cars and look at THOSE numbers. Take him to an insurance agency and let him look at the cost of THAT. Take him all over town to put out applications for a job. He should see what the salary is. Then plug the numbers into a calculator so he can see what FICA and taxes will take and what he will end up with. Deduct apartment, car and insurance from THAT. Unless he is an absolute dummy, he will "get" it.
You should be demanding that he get a job this summer. Spend every single minute driving him everywhere to put in applications. Make him think like an adult if he wants to "play" adult with this girlfriend.
And be nice to her parents. They are evidently struggling too, and Lord knows what they think of YOUR son, by the way. If that mother were here on MP, I can only imagine the stuff she would say about the boy and about you trashing her daughter.
Well, your son is just as complicit in the behaviors as she is. Why are you giving your son a break here? This really shouldn't be about her. This should be about HIM. HE is the one you can't trust. And he is the one who is flaunting your rules.
You have to learn, Mama, that trying to say no to their relationship makes it forbidden fruit. Unlike the girls' mom, you can't go to the doctor and get depo-provera shots. You can't proactively prevent pregnancy. But you can actually push these two into having sex if you do this "watch him like a hawk" thing.
Re-think your strategy. Instead of thinking about how you can get rid of her, think about how you can get your son to start respecting the rules of your home in regards to his guests. Instead of this being about HER, it should be about ANY girl he brings over.
I told my son that bedrooms were off limits. They can be in the living room by themselves - we will go upstairs and give them privacy. He was told that if we let his girlfriend go in his bedroom, then we'd have to allow it with his younger brother (who actually isn't dating yet anyway). And we stuck to that. I also told my son that he needed to understand that her mother needed to respect him, and that meant behaving appropriately in word and deed when over at her house. Interestingly enough, the girlfriend's mom allowed them in her bedroom (with the door open). When he brought that up, I reminded him that I couldn't concern myself with her rules - I knew that the rules I had set forth in my house were what we needed.
I also talked a lot about personal responsibility in terms of pregnancy and STD's. AND the fact that child support goes on and on and on, and I wouldn't pay it for him.
I let him know that I trusted his judgment. He never gave me cause to not trust it.
You obviously don't trust your son. But I think that you are basing this mistrust on HER instead of him.
It might help you to treat her as if she were your daughter instead of a guest. You should have clear rules. "Susie, this bathroom is the one I want you to use. Please wear your bathing suit in the pool instead of regular clothes." And stop worrying about her wearing his clothes. She enjoys it because it makes her feel good. If you can get her out of his bathroom and bedroom and have her wear a bathing suit in the pool, two out of three "ain't bad". Sometimes you have to pick your battles.
Tell her what you want help with in the kitchen. Tell your SON to help you in the kitchen. Have them both clean the dishes when dinner is done.
However, you should TRY to make it comfortable in your house for them, a place without all this family strain so that they will want to stay. It's better than them going out somewhere else (sneaking around, as you put it.)
Look, he is 17. You are acting like he is 15 or younger, not wanting him to talk to her. Kids today text and talk to each other all day long. (Yeah, I've said to my husband, "What on EARTH do they find to text about!" And then he reminded me of when he and I used to talk on the phone and fall asleep talking to each other, neither of us wanting to be the one who hung up first!)
I hope you can see the truth of the matter that the more you push against her, the more you will make your son want to be with her. When he turns 18, he can do what he wants - you may find that all of a sudden he turns up married to her and living with her folks. And then you will have a daughter-in-law, and bad relations from the start that will take years, or maybe forever, to fix.