Give all ~15-year-olds a day-long standardized test. The test is mostly a bunch of boring "do lots of not-hard math", "read dull stuff & kinda understand it", etc. Kids are encouraged to bring their phones - but can't use them in any way that plausibly amounts to cheating on the test.

Kids with good scores on the test get admitted to high school. Ones with not-so-good scores (whether really dim, horribly prepared, or too addicted to their cell phones to pay attention) go to a menial labor training camp. Where they get to post pictures of the ditches they're learning to dig, potatoes they're learning to harvest, bathroom fixtures they're learning to clean, etc. all over their social media accounts.