The key is you have to pass a comprehension test. And it was only a 4 day reduction, for every 30 days spent, max. Less than a 12.5% reduction.
Yup, and the comprehension test is ministered by the Judge who's in charge of processing *all* of the inmates pleas for reductions, paroles, etc... So i'd bet he's going to be pretty thorough (although condescending, based on the average education level of the inmates), before giving them a "pass".
personally, i agree with Nanshork. I don't see it making much of a difference. It would be different if there was mandatory education for all inmates. They don't get out before they have their highschool diploma, at least. This, i wager, would have an impact. They're not doing anything, as is, and this way, at least they'll have some CHANCE of getting something better when they do eventually leave prison.