Not only would it be a giant waste of money, we can't get anywhere near close to all our population to stay in school through what we currently offer, why would they go any further? Also, some people do not need to go college because they are doing something like working and then taking over a family gas station.
Tough truancy laws. Better funded schools and teachers that care when students don't show up.
And are you suggesting that people don't go to college because they don't want to and would rather work? Family gas station? What is this? 1956?
The vast majority of those who don't go to college don't go because they can't afford it not because they'd rather work off of food stamps and minimum wage salary. If it's free, they will go.
There are plently of people who are doing jobs that don't require a college degree, compulsory college is an absolutley terrible idea. Again, ideal society would keep people in school but there is no way inner city schools are going to keep down their drop out rates to low levels.