MONTPELIER — The state Supreme Court ruled Friday that a former Castleton man convicted of killing a school janitor in 1992 is entitled to a public defender to continue his appeals, even though attorneys who have looked at his case ­believe his legal claims have no merit.

The decision came on the 20th anniversary of the killing of a Castleton school janitor by Charles Crannell, now 70 and serving a life sentence in a ­Kentucky prison. The decision was in response to an appeal filed by Crannell, who argued he was entitled to a lawyer provided by the state because his conviction predated a 2004 law that forbids attorneys from filing ‘‘frivolous’’ legal cases.