Can the school reform movement accept constructive criticism? Gary Rubinstein hopes so. Mr. Rubinstein joined Teach for America in 1991, the program’s second year, and has now been teaching math for 15 years, five of them in some of the nation’s neediest public schools and 10 more at the prestigious Stuyvesant High School in Manhattan. He has a bachelor’s degree in math and a master’s in computer science, has written two books on classroom practice and at one point helped train new corps members for Teach for America. For years, he was a proponent of the program, albeit one with the occasional quibble.