Students walk out of the Arizona Preparatory Academy location at a strip mall in Phoenix.

Alternative schools can be the last best hope for students who have been expelled, can’t succeed in a traditional classroom or fall behind their peers.

In Arizona, more than 180 alternative schools help these at-risk students catch up on credits and, hopefully, acquire a diploma. The latest accountability data show these schools are improving.

When the state Department of Education released letter grades for schools earlier this month, nine alternative schools received the highest mark — A-alt. That’s the same number as last year. However, because the schools performed better overall, the department toughened criteria for earning an A.

“This is empirical evidence that schooling for non-traditional students continues to improve,” said Amy Schlessman, president of the Arizona Alternative Education Consortium, which includes about two-thirds of the schools as members.

“All alternative schools in the state should get some recognition that ‘Hey, you guys all got better,’ and, in a sense, that’s what letter grades are all about. It’s helped us to continuously improve, and the students are the ones who benefit from that.”

About 28,000 students are in alternative education, about 3 percent of the total in Arizona’s public schools.

Like traditional district and charter schools, alternative schools are scored based on results of the Arizona’s Instrument to Measure Standards test. But because they serve a unique group of students, the state scores alternative schools differently, with less weight given to the actual AIMS results and more weight placed on academic improvement.

The average total score for the state’s alternative schools for 2013 was about 125, compared with about 120 for 2012, according to state Department of Education data.