They should have a work program where jobs are reserved for people who have the dedication to continue to go to school at 19 and 20 but just cannot grasp the neccessary concepts to graduate.
If at 20 you are unable to graduate high school I feel you should be provided special oppritunities or programs to help similar to a disabled person or veteran.
Anyone that doesn't care or is simply refusing to do the work is not gonna hang around after 18 when it isn't required anymore.
Realistically, they drop out earliar if they just want to hang out and sell drugs or whatnot.
Someone still in school at 19 and 20 is trying, they just either have something going on mentally, physically, or in their enviornment that is keeping them from advancing. They need help.