The Only Thing You Know (1972)

You thought living through the 60s was hard? Here come the 70s....Freedoms have
been gained, but new challenges abound. For Ann, an 18 year-old high school
student, the promise of social independence comes alongside the responsibility for
one's choices. A chance encounter with Scott, a twenty-something high school
teacher, leads Ann into a relationship that causes friction at home and with her
closest friend Laura. When Ann leaves home to live with Scott, she discovers that
he has recently lent a couch to an American interloper, Paul Evansfree. In the
cramped space of Scott's apartment, the trio must come to terms with one another
and with their desires. Faced with pressure from home and from Laura, Ann must
decide where her future lies.

http://www.imdb.com/title/tt0067534/