I think that it should be legal to drop out of high school. If parents have unstable conditions in the family, for example in debts or having problems to even afford three meals for the family, dropping out of high school would definitely be necessary for them. Although it's not recommended to drop out of high school, it should be completely legal as families need to make decisions which are sensible and the best for them.
So many kids are dropping out of high school with very little education. Many jobs require a high school GPA now. If kids keep dropping out then we will have no educated people to do the jobs that require you to go to college. High school is important in today's society.
Students under 18 shouldn't be able to drop out of high school, however there should be an option to test out of all the subjects required for graduation, and that should allow students to leave compulsory education before age 18. Parents would have to approve though because of the age
A diploma is a social construct
For many jobs, the only reason why you need a certain level of education is because it shows that you have a certain level of intelligence, are reasonably well adjusted and tend to conform to authority. The actual things you learned in school aren't relevant.
Since education is used as a selection mechanism (and a very expensive one at that), it doesn't matter what absolute level you have, what matters is how your level of eduation compares to that of everyone else. This means that forcing everybody to finish high school will only have the effect that the amount of education you'd need for a certain type of job will only increase. It any easier for the bottom, say, 10% to find a job.