21 Jump Street's main message is that friendship trumps high school stereotypes and popularity contests -- as long as you keep an eye on what's important (relationships) and what's not (your position in an imposed hierarchy).
Lots of it, but generally played cartoonishly (and lots of slow-motion scenes). A character rejoices when his friend shoots someone (it's a gross-out moment); guns are drawn and pointed at someone's head. Though most of the fight scenes are cartoonish in nature, blood is shed. And cops rejoice after arresting and shooting people.
Three people are shown in a state of undress while in the middle of a sex act (though there's no graphic nudity); in an end-credits scenes, two couples are seen together. Also lots of crude language and sex talk, including discussion of teens hooking up and/or wanting to. Some kissing and sexual gestures; genitalia are drawn on family photographs and also can be glimpsed briefly in a non-sexual context.
Very frequent use of strong language, including "f--k," "s--t," "ass," "bitch," "p---y," "prick," "d--k," "damn," "hell," "goddamn," and "oh my God" (as an exclamation).
It's a teen movie, so the products that teens use -- laptops, phones, etc. -- are on display. But there's not a lot of label-dropping.
Drinking, Drugs & Smoking
The plot centers around the arrest of a drug ring that deals in a high school; they've created a synthetic drug whose effects have been chronicled on a website. The video has gone viral, and the 21 Jump Street agents have to stop the drug from spreading, too. There's also pot-smoking and some underage drinking at parties.