late 13c., "a beginning, act or fact of coming into existence," from Old French comencement "beginning, start" (Modern French commencement), from comencier "to begin, to start" (see commence).
Meaning "school graduation ceremony" attested by 1850, American English, originally in colleges, in reference to the ceremonies by which members of the graduating class are made ("begin to be") bachelors, masters, etc. (commencement in the sense of "entrance upon the privileges of a master or doctor in a university" is from late 14c.) ; thence extended to graduation ceremonies of academies and lower schools.
I know what you are thinking of — the class members grouped in a semicircle on the stage, the three scared boys in new ready-made black suits, the seventeen pretty girls in fluffy white dresses (the gowns of the year), each senior holding a ribbon-tied manuscript bulging with thoughts on "Beyond the Alps Lies Italy," "Our Ship is Launched — Whither Shall it Sail?" and similar topics. [Charles Moreau Harger, "The Real Commencement," New Outlook, May 8, 1909]