twelfth grade n (Education) (in the US) the final year of secondary school after which students usually graduate at age 17 or 18 ?twelfth-?grader nCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014