the first draft lottery was in dec 1969 before that no lottery..before the lottery everyone had no idea if they would be drafted or not, but could assume so if they ended their deferments..dropping out of school usually meant "reporting for duty" ....also deferments changed due to its unfairness...take a look at the movie Big Wednesday, its a surf movie but has some very realistic scenes about the era..
I graduated in 1961. There was an active draft, even though we were not yet seriously committed to Viet Nam--it was simply a draftee army in those days.
I knew I would be getting a draft notice to report for a physical--and the likelihood was that you'd be inducted--so I volunteered for the CIA and the Air Force officer program