As a fan of teen comedies, I do think about this. If everyone's going to look 25 and talk in this mature way, why is it even set in a high-school? The two factors I see are:

Once upon a time : by setting a story in a non-realistic / mythic setting, it's easier to enter into the fiction of it. For adults, it has a nostalgia for a time before responsibilities when everything was possible, but that would be ruined if you had to face up to how akward and useless most teens really are. And for kids these 'teens' who look perfect and always know what to say are wish fulfillment. Everyone knows it's not really like high-school, but peasants and the aristocracy knew that knights were nothing like those dipicted in chavalric ballads, but they both like to imagine that they were for different reasons.

Bottle episode : High school is a super convient writing drvixe, because you have these characters who have freedom and independence enough to move the story forward, but it's also super easy to restrict any option that makes things difficult. There's no need to worry about too many social circles, or why the characters don't just do x or y. If you want a group of friends, who basically only interact with each other, it's plausible enough. Even in college that's harder to do, unless it's a very small, exclusive group (like The Secret History) and even then it feels intentionally insular and incestuous in a way that a high-school clique doesn't.