my first thought would be work harder and get better grades, so i could've had a head start on my career. maybe i wouldnt' have been unemployed for the first six months out of school then, and i wouldn't have bounced around from job to job trying to see where i really fit in.
but had i had a great job right out of school, i might've never been disillusioned enough to quit and travel, to see and have life changing experiences all over the world, and come back a changed person. career-wise, i've landed on my feet and am doing fairly well for myself now, and even though i'm a couple rungs behind people my age on the corporate ladder, i dont consider those years wasted.
not that i'm advocating goofing off in school, just in my case, i dont think i'd change a thing.