This is just my opinion, But I don't believe that not graduating college should be made illegal.
I have not been to college yet, But it seems that college is challenging and requires a certain intellectual level and a reasonably good educational background. If some secondary school students struggled tremendously in their secondary school classes and even with hard work and good srudy habits obtained below average (D) or lower grades, Are they really ready for or suited to college at the academic level?
There are youth with severe brain damage or intellectual disability who seem to lack the mental capacity to go to college, Unless someone creates a post-secondary progam for students with severe neurological and mental disabilities.
Also, Most colleges are competitive and have specific admission requirements. Not all students meet those admission requirements, Which means not everyone will get into university, Making it difficult for graduating university to be made illegal.
If by "college" you mean education for youth 16-18 years, Then to me it makes more sense for it to be a requirement. I also think that making some sort of postsecondary training including vocational education and trades a requirement makes more sense than making purely academic university courses a requirement, Because some students may not be well suited to or may not want to do academic study at university level.
If graduating university was illegal, A lot of people may be unable for whatever reason to do so, And thus would break a law, Unless the university graduates even people who fail in the courses. I don't believe graduating university should be made illegal simply because I do not believe everyone has the resources or has the capability to graduate univetsity, And because I think high school graduates should be allowed to choose more vocational study or a career.