Smart, interested 15s sure. The kind of folks you mention, why would they even come? Or not drop after the first test.
Why would they ever go to high school to begin with? Why not just drop? Same reason they all go - societal pressure.
Besides, being a disruption or a PITA does not mean you fail academically. You can do quite well academically but absolutely be a huge PITA to all your peers.
I think you overestimate the intellect and interest required to succeed in an academic environment. We don't live in some weird society where only the most intellectually gifted (however you define that) and most interested are capable of succeeding in college or academica.
Truancy laws aren’t societal pressure. They’re a threat, enforced, that if you don’t send your child to school men with guns will lock you up.
Disruptions and PITA do not want to be in school, for the large majority. For all of them, they do not want to be in that class. If they were free not to go they wouldn’t.
You’re quite right that standards have been dropped enormously since when academia was the pursuit of a small minority but they haven’t slipped to where someone of average intelligence can graduate from a four year university. Standards still exist.
I don’t know what you think intelligence is but I’m gonna say this is easily disproved by the number of dumb as dumbbells folks I see out there with degrees...
Truancy laws only get enforced in certain regions and vary on rules... I saw plenty of kids dropping out where I grew up.
Dumb for a university graduate: can’t do algebra
Dumb for a high school graduate: can’t divide a five digit number by a two digit number with pencil and paper
Dumb for the general population: can’t do arithmetic
Having majored in music (and picked up most of my technical skills just out of a habit for tinkering), my running joke was always that none of my classes required me to count past 32.