Kiefer Sutherland to Charlie Sheen -- At Least ONE of Us Has a Diploma!

Kiefer Sutherland has nothing but respect for his "Young Guns" co-star Charlie Sheen ... who finally got his high school diploma this week ... because it's something Kiefer doesn't have.

Kiefer -- dapperly dressed in a suit and fedora -- was outside Madeo in LA last night ... and may have been reliving his own high school memories when he told us it was "fantastic" that Charlie got his diploma -- 30 years after the rest of his class.

Compared to Kiefer, Charlie's something of an overachiever. Kiefer himself attended several different high schools as a kid ... but revealed in interviews, he never got his diploma.