Actually, Ferris' high school, Glenbrook North High School [1], is scheduled to get out on June 9th this year [2]. School calendars usually follow similar schedules over many years, so it's reasonable to assume that it had a similar schedule in 1985, making the last scheduled day either June 6th or 13th. Plus, snow days invariably cause the school year to go a few days longer.

[1] http://www.imdb.com/title/tt0091042/trivia [2] http://www.glenbrook225.org/north/calendars/Documents/2010_2...