There was a 4 year cap up through the 8th grade (so I was a 4th grader attending 8th grade English, math and science classes), and after that, the cap was lifted and students could begin taking college courses in 8th grade --- some of the teachers were accredited as faculty at a nearby college, and if need be, arrangements were made for students to travel to the college, or professors from there to travel to the school.
It was not uncommon for students to graduate from high school and simultaneously be awarded a 4 year college degree.
Apparently, the Mississippi State Supreme Court declared the system illegal because it conferred an unseemly advantage on the students who were able to take advantage of it, with no equivalent compensation for students who were not.