Wisconsin’s largest teachers union has filed a second lawsuit asking for the closing of a virtual charter school it says violates state law.

The Wisconsin Education Association Council filed the lawsuit in January against the Northern Ozaukee School District. The district operates the Wisconsin Virtual Academy in conjunction with K12 Inc., a for-profit company led by former U.S. Education Secretary Bill Bennett.

The academy, which opened last fall, has 420 students in kindergarten through seventh grade, and most of them live outside the district.

The lawsuit is the union’s latest challenge to virtual charter schools that use state money to support the home-based instruction of non-resident students.

The suit seeks an injunction to close the school. It also asks the court to rule that such schools are illegal and that state aid payments to the district on behalf of virtual school students should be withheld.

According to the suit, open enrollment aid is based on the assumption that students will physically attend a school in another district. The suit argues a

virtual school’s per-pupil costs are likely lower than traditional schools’ expenses because virtual schools have fewer teachers, support services, and other programs.

Northern Ozaukee Superintendent Bill Harbron defended the academy and said the school is taking advantage of laws meant to change public education.

The union’s lawsuit against another virtual school, Connections Academy, was dismissed last year, though the union has appealed. Similar legal challenges in Pennsylvania also have failed.

Wisconsin has at least six other virtual schools, and more are expected to open.