(CBS News) Sal Khan is a math, science, and history teacher to millions of students, yet none have ever seen his face. Khan is the voice and brains behind Khan Academy, a free online tutoring site ...

The following script is from "Teacher to the World" which aired on March 11, 2012, and was rebroadcast on Sept. 1, 2013. Sanjay Gupta is the correspondent. Denise Schrier Cetta, producer. Take a ...