As part of its biannual report on the prison system, the board calculated enrollment in academic classes at 35.9 percent of existing capacity. That means that two-thirds of the classroom slots available for adult inmates in California were empty as of last June, when the data was compiled.

The California Department of Corrections and Rehabilitation has placed a premium on educational, vocational and drug rehab programs to help satisfy a judge's order to drastically reduce the inmate population. At the same time, the department has had to slash $250 million from adult inmate programming as part of wider cuts to the state budget.

Earlier this year officials said they were "streamlining" programs that were “proven to reduce recidivism,” in part by giving priority placement to inmates close to release. Special classes help inmates develop basic literacy skills and obtain high school diplomas.

But it appears that effort may be flagging, leaving thousands of inmates shut out of the classroom.

The oversight board said it was trying to determine if the low enrollment was “simply a result of programming startup as the department asserts, a broader systemic concern, or is concentrated in select institutions.”

Last March the board questioned [PDF] whether the department could “realistically provide rehabilitative programming resulting in reduced recidivism” after “laying off approximately 800 teachers, severely restricting the time spent in class, eliminating many vocational programs and cutting in-prison substance abuse programming to 90 days.”

Here's a breakdown of five California prisons with the lowest enrollment figures for academic programs (measured as a percentage of available slots):

• North Kern State Prison: 7.2 percent
• R.J. Donovan Correctional Facility: 17.5 percent
• California State Prison, Sacramento: 17.7 percent
• California State Prison, Corcoran: 17.8 percent
• California Correctional Institution: 18.5 percent

The Sierra Conservation Center in Jamestown ranked highest at 70.1 percent.