Given the journal, that seems like an odd error to make. I could have that wrong, but it would make sense given that article specifically mentions truancy and underage drinking, two charges which seldom result in arrest.
Maybe that makes the statistic a little less shocking to those surprised by the number? Again, I certainly could be wrong, but it really would be surprising if the 40% refers to people actually having been read Miranda rights, hand cuffed, placed in a police car, etcetera. Even for other common crimes for that age group, like possession (generally marijuana), usually a citation is issued and the drugs are seized but no arrest is made. This is all said from the perspective of a judicial clerk in Portland, Oregon, who works a lot with the dockets related to citations related to minor misdemeanor offenses. Perhaps it is different in other states.
Anecdotally: almost everyone I know well has been convicted of a misdemeanor of some sort for which they could have been arrested but only one person I know has ever been taken in to police custody.