The Marine Corps does not allow you to do this, but the Army does. You initially enlist in the reserves and go to basic in between your Junior and Senior year. You can transfer to the active component when you graduate. After you graduate, you go to AIT (advanced individual training) and earn your military occupational skill. I believe that the situation that you describes was about a young man who had an early exit from high school. He had his diploma in December and then shipped to basic in the spring. The fight was not over being to wear his uniform, rather it was that he had to put his graduation gown over it. He was allowed to wear it in the beginning of the ceremony for presentation of the flag, but he was required to be in the school's uniform of the day which was a gown. The school was trying to maintain uniformity just as the military does. I was in ROTC and on graduation day I was commissioned. Commissioning was scheduled before the graduation so I wore my uniform for commissioning and wore my gown over top of my uniform. Being a Marine, he should understand what the uniform of the day is.
Anyway, good luck.