1 adj. — Education
characterizing a scheduled block of free time for either students or teachers, usually in a high school.
Type: 5. Frequency — The adjectival use of the term, most often in the phrase "spare period," precedes the noun use, as in "I have a spare after French class" (see meaning 2). This usage occurs most frequently in Canada (see Chart 1). In the US, as in some other locations, the more common term appears to be "free period", which is also used, albeit as a minority term, in Canada (see Chart 2). The term is a preservation from British English (see the 1942 quotation).
See also: bird course semestering
2 n. — Education
a period of the school day in which a student or teacher has free time.
Type: 5. Frequency — It appears that spare as a noun was derived from the adjectival form (see meaning 1).
See also COD-2, s.v. "spare"(3), which is marked "Cdn".
See also: bird course semestering
Images:

Chart 1: Internet Domain Search, 15 Oct. 2012 
Chart 2: Internet Domain Search, 4 Jul. 2013