California per-pupil spending 2008: $9,015 California’s Annual Costs to Incarcerate an Inmate in Prison: $47,102Why should those be, in any way, comparable?
Prisoners need cells, three meals a day, healthcare, clothing, and constant supervision, 24 hours a day. Schoolchildren only need classrooms and supervision for six hours a day five days a week. And prisons can't get away with a 30:1 inmate-to-guard ratio, either. It makes perfect sense that a prisoner should cost more than a schoolkid.
Of course I'm sure California prison guards are still ridiculously overpaid. My personal solution to this conundrum is to outsource California's prisons to Mexico.