I graduated from high school in '71 and proceeded on to NCSU, which happens to be when Edwards was there. The details escape me, but as I remember, the college deferment was dropped when the "lotto" type selection was instituted. By that, I mean, every day of the year was printed on some pingpong balls, and were removed randomly from a bin, one by one. The date of your birth (if you were a male and physically eligible)was assigned a number that matched the sequential order that it was drawn from the hopper. That number became your draft number.
Mine was something like 327, which, at the time, was too high to be drafted. I stayed in college, but I know at least one classmate who volunteered, after his birth date came up in the low numbers. Maybe someone else can be more specific about the draft policy at the time, but this is how I remember it.