session vs class period

session and class period both are nouns.

Word	Noun	Adjective	Verb	Adverb
session	Yes	No	No	No
class period	Yes	No	No	No

As nouns, class period is a hyponym of session; that is, class period is a word with a more specific, narrower meaning than session:

Other hyponyms of session include course session, recitation.

session (noun)	class period (noun)
a meeting devoted to a particular activity	a regularly scheduled session as part of a course of study
a meeting for execution of a group's functions
a meeting of spiritualists
the time during which a school holds classes