The state’s high court ruled Thursday that people convicted of murder as teenagers are not being treated fairly by the Department of Correction when prison administrators are deciding whether to allow them to serve time in minimum-security prisons.

In a unanimous ruling, the Supreme Judicial Court ordered the department to change its rules and comply with a 2014 law that instructed the prison system to let juvenile killers – regardless of their current age – move to lower-security prisons. The ruling comes after the SJC upheld the law’s legality last year. “We conclude that the department still falls short of the requirements’’ of the 2014 law, Justice Kimberly S. Budd wrote for the court. “The department continues to block the majority of objectively qualifying juvenile homicide offenders from placement in a minimum security facility.’’

The court ordered the department to record in-prison hearings where inmates can challenge classification decisions made by staffers that determine whether an inmate serves a sentence in maximum-, medium-, or minimum-security prisons.

Under a two-step system, the court said, 42 juvenile killers initially were cleared for transfer to minimum-security prisons, but officials at the next step invoked vaguely worded rules and required some 90 percent of the inmates to remain in medium-security prisons.

Only 5 to 15 percent of other inmates are required to remain in their existing security setting, the court said. “In practice, the process still largely deprives the inmate of individualized review,’’ Budd wrote.

The new rules apply to anyone convicted of committing second-degree or first-degree murder prior to turning 18.

John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.