When you registered for the draft, as was required, you were first sent a draft card classifying you as 1H, or holding. Then they drew the lottery for guys born in your birth year. My number was 087. I got a new draft card in the mail re-classifying me 1A, available, within a week. I can still remember picking up that notice from Uncle Sam in my mailbox at the Poteat House post office.
I called the draft board to ask why I was not 2S, college deferment. I was told that the 2S classification had been eliminated. But it was still listed as a classification on the back of the draft card. Freshmen guys on campus were either dancing for joy, nervous, or scared shitless, like me.
Although, I will add that by the time I was 18, the guys with the low draft numbers were more nervous about what was going on in the Middle East then in Vietnam.