Another thing that struck me about 21 Jump Street is how effective the script was at creating memorable supporting characters, even if a few of them only pop up for a scene or two. This is one of those things that can elevate a screenplay and often show the difference between a seasoned pro and a newbie.
There's an early scene where Parks & Recreation's Nick Offerman plays the typical world-weary police captain who has to reprimand the main characters. In most scripts, this character would just be there to yell at the guys, transfer them to a new division and then throw them out of his office. It could have been purely an A-to-B plot moving scene, but the script does a little bit more with it. Through his interactions with Jonah Hill and Channing Tatum, Offerman creates a character who feels three-dimensional. Some of this credit goes to Offerman, but the writing gives him plenty to work with.
Offerman gets one of the best lines in the movie when he says that the Jump Street program is just another idea from the 80s that's been revived because no one has any original ideas anymore. There's a sort of bitterness in his tone, and it sells the verisimilitude of a line that easily could have reeked of meta-humor and nothing more. You buy this guy as someone who's counting the hours to retirement.
Another good example - the science teacher played by The Office's Ellie Kemper. She's pretty much the only character in the high school to remark on Channing Tatum's attractiveness and it makes for an awkwardly funny scene as she trips over her tongue. Is the scene necessary? Does it really move the story forward? No, but it's unexpected and makes for a funny moment.
Weak scripts only worry about making the main characters pop off the page. Excellent scripts make every character memorable and distinct. Always push yourself to make these smaller roles pop. Write each part as if it could be a cameo by a gifted comedy star or a strong character actor.