The U.S. Supreme Court has declined to review whether a 2015 lawsuit alleging gross failures in the foster-care system should be treated as a class-action matter.

The high court decision means the case will proceed to trial as a class-action lawsuit, the Arizona Republic reported.

A trial date has not yet been scheduled before U.S. District Court Judge Roslyn Silver. Any changes to the system resulting for the lawsuit will apply to all children in Arizona foster care, as well as those in the future.

Attorneys for the Arizona Department of Child Safety and the state's Medicare provider argue the lawsuit conflates problems that individual children have encountered with systemwide failures.

New York-based nonprofit Children's Rights has brought similar lawsuits in other states, arguing the problems are systemic and can only be solved with judicial intervention.

Silver's decision to classify the matter as a class action was upheld by the 9th U.S. Circuit Court of Appeals.

The Department of Child Safety has argued that it has made substantial improvements, citing a 23% decrease of children in state custody and to a higher rate of children leaving the system, either because the children were returned to their parents or were adopted.