Boston school superintendent Carol Johnson can’t transform public education by extending the school day for 30 minutes. She should abandon her months-long effort to cut a deal with the Boston Teachers Union for working an additional half hour. It’s not worth the money or the aggravation. Education experts from the National Center on Time Learning were in town earlier this week for a national conference that highlighted how more time in school translates into big achievement gains for urban students. Successful efforts often require adding two, three, or even more hours of academic and enrichment programs to the school day. That’s what it takes to close the achievement gap between low-income, urban students and their suburban counterparts. Not a paltry 30 minutes.