From the Super Mario Wiki
“You'll never be plumbers! That's what they'll put on your tombstones!”
Sergeant Kooperman is a large, overweight, strict and sadistic drain sergeant who lives in Brooklyn, where he teaches at the Brooklyn Plumbers Academy. Coincidentally, his voice is exactly the same as King Koopa's.
When Mario and Luigi first arrived at the academy, they entered Kooperman's classroom, where the sergeant took an immediate disliking to the two of them, due to their unimpressive physiques like their short statures and their fear of him. He told them that they had the wrong stuff, which surprisingly, Mario seemed to take to heart a bit. Sergeant Kooperman put Mario and Luigi through several back-breaking rigorous tests, the first of which involved the brothers to each carry a bathtub up the fire escape of a tall building. Unimpressed with their time, Sergeant Kooperman forced the two of them to take the tubs back down and start all over again without even permitting them to take a brief break.
For their next test, Sergeant Kooperman made Mario and Luigi each unclog a sink within a ten second time limit. By accident, some of the sludge that Mario and Luigi were unclogging splashed onto Sergeant Kooperman's face. Sergeant Kooperman was angered at Mario and Luigi even more so, he put the two of them on K.P. (Killer Plumbing).
As part of their K.P. Mario and Luigi were then forced to dig a very deep hole using shovels. When Mario requested a glass of water, Kooperman rejected the request, and put Mario and Luigi on a different assignment; fitting a mess of assorted pipes together in twenty minutes with no wrenches. After Mario and Luigi attempted to put the pipes together, Sergeant Kooperman put them to another test by blasting water at them with a hose; the water caused their pipes to collapse, trapping them inside, with the water level rising. Sergeant Kooperman refused to help "guys with the wrong stuff", and made things worse by tossing more pipes on top of them, telling Mario and Luigi that they'd never get to be plumbers.
After Mario and Luigi got out of the pipes, they left Sergeant Kooperman, as well as the academy, and considered a different career option. Meanwhile, Sergeant Kooperman went on a plumbing job of his own, where he kindly agreed to unclog a woman's sink. When Sergeant Kooperman went outside to pick up a sack of Drain Buster, he accidentally picked up a sack of concrete that some nearby construction workers needed instead.
Pouring the concrete down the sink, Sergeant Kooperman assumed that the problem was coming from outside, where two of the construction workers question him as to why he's pouring their concrete down a manhole. This action clogged the sewers, which in turn caused the streets to start flooding, just as the U.S. President and the Russian President were being driven around Brooklyn.
Although Sergeant Kooperman was never seen again, Mario and Luigi managed to unclog the cement themselves, saving the two presidents from being washed away. For their bravery, the President awarded Mario with a Golden Plumber's Helper, and Luigi with a Golden Wrench.