trying to help someone lose weight by tweaking their scale to always show a healthy BMI.

Assuming the scale is functional. Which is is not.

It seems like there’s almost certainly a confounding variable here: the kids who are likely to engage in criminality are also the ones most likely to do poorly in school, skip classes, and be held back.

You're conclusion doesn't follow. Case in point: kids who are kicked out of school are more likely to become criminals because they are children and have potentially lost the only thing remaining in their lives that kept them on any path of any kind. And they don't have to start dealing drugs and robbing people with all the extra time they have. A kid who has dropped out or been expelled can still be charged with truancy in most jurisdictions, and their parents charged with the same crime and fined (up to $1,000 in many places). An underage child can be taken from their parents based on truancy violations alone. Then there's loitering, trespassing, panhandling, and a whole mess of other non-violent offenses that give a high school-aged child a criminal record.

A person isn't a criminal until they've committed a crime. They aren't a convict until they've been found guilty of crime. Most of the kids being expelled, suspended, sent to Alternative Learning "SuperMAX" Centers are non-violent offenders. They are put out because they can't behave.

The argument from here is often that we have to put the undesirables somewhere. After all, it's not fair to sacrifice the education of the other, well-behaved students. I agree. I also assume that most people would want to help these children. On that assumption, I'll finish this post with two bits of info:

• COPS In Schools (CIS) was created after the Columbine shootings. The grant appropriated $750-million to hiring police offers (School Resource Officers or SROs) for placement in schools. Despite their fancy title, these are police officers and they can and do, at a frightening rate, arrest children.
• In 2022, a bipartisan bill, the 2022 Bipartisan Safe Communities Act, was enacted. It granted $1-billion dollars to the nationwide development of mental health resources (counselors, social workers, programs) in schools, and the entire country rejoiced. In 2024, the Trump admin cut the grant because of DEI.

We've chosen to police children instead of help them.