Use of the word "freak" as a slang term to mean stoner or heavy marijuana user

Freak was common in the 70s and I was in public school.

"Freak" in slang usage connotes sexual activity or kinky sex. Urban Dictionary

To answer number one: Steely Dan has a song on their album Pretzel Logic called Charlie Freak. The song revolves around a man who sells everything he owns for drugs. So, with this example, we see that the word was used beyond boarding schools. (The album was recorded in New York, 1974.)

EDIT: Added year of song

I can only speak of my own experience as a high schooler in 2 Chicago suburbs from 1969-1973, where “freak” was widely used to mean not just stoners but generally anyone who identified with hippy counterculture as expressed in dress, hair, speech, politics, etc. The term seemed to gradually fade away in the mid seventies as all these manifestations entered the mainstream and no longer functioned as subculture identifiers.

In short,

1. the noun “freak” was first recorded in the 16th century and meant a sudden whim.
2. It then became an irrational characteristic, then
3. eccentric behaviour, then
4. anything that was eccentric; then
5. anything that was curiously physically abnormal, then
6. anyone who took an abnormal or obsessive interest in anything

Most of the meanings are extant.

According to the OED,

Freak

Around the beginning of the 20th century, the meaning of freak developed to describe someone who was and enthusiast for something – the term could be either derogatory or neutral.

d. With qualifying word or phrase: one who shows great enthusiasm for the activity, person, or thing specified, as health freak, train freak, etc.; an aficionado. Cf. buff n.2 6b and fiend n. 4c colloquial (originally U.S.).

1908 Court of Appeals, State of N.Y. III. 455 He had a camera. Evidently from the evidence in this case he was one of your kodak freaks.

1959 L. Lipton Holy Barbarians i. 39 He looked more like one of those beachcomber Nature Boy health freaks than a real hipster.

Probably because of the rise in drug use in the 1960s, freak took about 60 years to be recorded in the way described, i.e. someone who is addicted or a regular user of drugs – often to the exclusion of most other things and with the concomitant lifestyle. (The earlier word had often been "fiend")

c. One who ‘freaks out’ (freak v. 3); a drug addict (see also quotes.).

[Given the history of the noun, the origin from “freak out” is uncertain – which came first?]

1967 Avatar (Boston) 1–14 Sept. 17/1 The life expectancy of the average speed-freak..is less than five years.

1969 R. R. Lingeman Drugs from A to Z 79 Freak... One who prefers a certain kind of drug, as in acid freak or meth freak... By extension, one who is obsessed with a certain way of thinking as in ‘political freak’.

Where the context makes it clear, the qualifying noun is not required.