A yojijukugo for going back to school.
Literally: warm – reason/cause/therefore – know – new
Alternately: Learning from the past. Gaining new knowledge or wisdom through (thorough) study of the past. Moving forward with new ideas, based on studying the past.
Note: It would seem logical to use 古 (meaning “old,” and also pronounced ko) in place of 故, but this is incorrect. The roots of the phrase are actually in a Confucian pronouncement which uses the character 故.