I am a high school teacher and I hate this look, but there is only so much I can do. A cafeteria worker was yelling one day that these students look like hoodlums and the teachers aren't doing anything about it. Now, we try, but when parents approve, the battle becomes difficult. Just as a pp said, it is not appropriate for a work environment, which is what we are hoping to ready our young men and women for-- and I worry that a child will trip and some parent will try to sue the school when his crotch was at his knees.
Anyway, my rule is NO VISIBLE UNDERWEAR in my class. 9 times out of 10 a student will choose to pull his shirt over rather than de-sag. I should not have to see underwear while I am trying to teach Shakespeare, it takes the sense of respect away from the experience.
Side note, worse than sagging (to me) is when I see a child "with sag" who has worn the same underwear two days in a row. (ie: "Dr. Seuss boxers) I really can't wait till this trend is over.