Oct. 1 brings the start of the second phase of New York’s Raise the Age law, as the age of criminal responsibility rises to 18. Is the juvenile detention system ready?

The new law challenges that system, unaccustomed as it was to dealing with potentially dangerous adolescents who in the past had been sent to Rikers.

As The City reports, last year’s shift of 16-year-olds facing felony charges from criminal courts to Family Court and new Youth Courts was a bit rough, and there will be twice as many 17-year-olds.

If a judge opts to remand an accused teen, it’s to the Crossroads detention center, run by the Administration for Children’s Services, or to the Horizon juvenile center, run by the Correction Department.

In the first phase of RTA, the Mayor’s Management Report reveals, assaults against staffers rose sharply at Crossroads; and the union for the staff blames the new law. Other troubling signs include rises of 25% or more in the daily number of weapons and illegal substances confiscated from detainees.

The report doesn’t provide separate statistics for Horizon, but it likely saw the same issues. (Horizon, incidentally, will transfer to ACS management next year.)

ACS chief David Hansell says Crossroads will handle this year’s transition better, as it won’t be absorbing kids sent straight from Rikers. The agency also claims the rise in confiscations is the result of greater supervision.

On the other hand, ACS also had trouble at its Nicholas Scoppetta Children’s Center this year, as Family Court judges send some teens there — not for detention, but for shelter, as their families refused to take them in.

The Post reported in June on how the Scoppetta overcrowding created havoc. Union officials there also claim that adolescents are assaulting staff, with no real consequences for misbehavior and violence.

The Chief-Leader reports that ACS plans to implement a new behavior-management system, STRIVE, that rewards good behavior with snacks and more phone time. ACS notes that more serious infractions, such as assault, can still mean arrest and prosecution.

The juvenile-justice-reform movement is excited about the progress it’s achieving, and rightly so: Keeping 16- and now 17-year-olds out of Rikers is more humane. But some of these teens are serious trouble; handling them in less-restrictive environments poses serious risks for staff and for the other kids at these facilities.

Though the system’s had a year to prepare, don’t be surprised if there’s big trouble as Raise the Age kicks into high gear.