A $1 billion plan announced by President Obama yesterday would provide for the creation of a corps of exemplary “master teachers” in STEM subjects who would lend their expertise to mentoring other teachers in all 50 states. In exchange, each teacher would receive a salary boost of $20,000 annually. The program would begin with 50 master teachers, building up to 10,000 in four years. Master teachers, who would be identified and selected through a competitive process, would need to demonstrate superior content knowledge and proven effectiveness in teaching STEM subjects, among other criteria.  “We need to be sure that we’re identifying the master teachers on the basis of demonstrable results rather than experience or credentials,” said Thomas Kane, professor of education and economics at the Harvard Graduate School of Education.