An Indianapolis school district said Friday it suspended five teachers and another resigned amid an investigation into cheating on a state standardized biology exam at one of Indiana's largest high schools.

The Washington Township schools suspended one teacher at North Central High School for 15 days without pay, one for 10 days and three others for four days, the district said in a news release. A sixth teacher resigned.

The district said one teacher took notes on the exam and shared those notes with others. The district said it has submitted a final report on its investigation into the disclosures of questions on the state biology exam in 2009 and 2010 to Wes Bruce, the Indiana Department of Education's chief assessment officer.

The department continues to investigate the matter and has not determined yet whether to throw out affected students' test scores, spokesman Adam Baker said.

The department each year investigates "a handful" of allegations of similar disclosures on state standardized tests, Baker said.

"This is one of the most serious ones that we've seen," Baker said.

Washington Township said that in 2009 and 2010, a teacher took extensive handwritten notes on questions by looking over the shoulder of students while they took the test online. Students in subsequent years said the teacher quizzed them verbally on the material "and then students saw the same questions on the test," the district's news release said.

That teacher shared her notes with four other teachers who told investigators they did not use it in the classroom, but one shared the material with his spouse, an administrator in another district. Another North Central teacher who was an administrator at the time received reports of the breach but did not act on the information. The four who received the notes and the teacher-administrator were disciplined for not reporting the security breach to their department chair as they should have, the district said.

The teachers who received the notes have been ordered to turn them over to the school's principal to be destroyed, the district said.

"North Central High School is taking significant steps under district direction to review and tighten training and testing protocols," the district said.

Bruce, the Department of Education assessment officer, said last month that teachers are trained in testing procedures and should know that copying and sharing test questions isn't allowed.

Other states release their state tests publicly each year and don't reuse questions, but Indiana keeps most of the test confidential and it's common for questions to be repeated.