This is a terribly difficul situation. Once they see themselves as independent/over 18, it is almost impossible to put the "genie" back in the bottle.
Because you truly can't MAKE them, I would stress two things:
1. You are either in school or you are working/on your own/living on your own.
MAKE THEM realize that school is their job or else they need to go out, get a full time job and MOVE OUT.
Of course it is super easy to live at home and have NO adult responsibilities. But if they are saying "I'm 18 and I'm an adult, then give them two adult choices: Choose school or choose work. One comes with our support of free rent; the other, requires full emancipation.
this is not MEAN because you are not "throwing them out" but are giving them two choices - one to be a student, the other to live on their own. But what you won't accept is just living at home, being indulged, and not living up to either the responsibiilities of being a student nor of being a grown up economically.
The other thing is to keep making them seeing this as their choice: Have them talk to other adults/school counselors/an uncle/an advisor at PCC to let them think about college choices....to answer the question: "Do I want to be IN school and graduate or quit school and go to work?" Each of these boys needs to see it as a CHOICE point they are making. Dont make it so easy for them to choose just doing basically nothing.
You can't exactly motivate them to want to do something hard. So you should focus on the adult decision they have before them. What choice do YOU want to make for your life?
They may not both make the same choice. Right now, they are riding on what is natural for teens, the feeling of the "herd"...that they should be able to coast and live at home if other kids their age/tribe have gotten away with it.
Its such a dilemna for parents. Just make them make a conscious choice. If they skip school, tell them you expect to see them making job applications the next day and getting ready to move out. Have them sleep in their car for a night. This is only dangerous if they have friends with delinquent parents who will let them come live with them and do nothing at someone elses house.
Also: really encourage them that they CAN do this difficult homework and stuff. Get a tutor to sit down with them to do the assignments they think are too hard.Make them do homework with you for one hour every night, no matterw what. If they say it is all done, make them do extra.
It is worth it if you can get them both graduated.
So try to give carrots/rewards and also sticks to keep them in school. Keep trying to emphasize what will happen if they don't finish/don't graduate? What will their choices be in 5 short months? At that point, they have to either get a job or continue school and if they haven't graduated, what school can they get into?