http://en.wikipedia.org/wiki/M-1_visa
The M-1 visa is a type of student visa reserved for vocational and technical schools. To obtain an M-1 visa for traveling to the United States, a student must present a signed Form I-20 at a United States embassy or consulate in his home country. The I-20 is issued by a designated school official, typically the international student adviser, after the student has fulfilled a school's admissions requirements and presented proof of financial resources.
M-1 students are admitted into the United States for a fixed time period. When they cross the border, their I-94 departure cards are stamped with a date, unlike students with an F-1 visa. They may stay for the length of their training program plus any Optional Practical Training, plus a thirty-day grace period at the end of their training. Their stay may not exceed one year unless they are granted an extension for medical reasons. If a student violates his status by, for example, not maintaining a full course of study, he is not eligible for the grace period.
Students in M-1 status may not work on or off campus while studying, and they may not change their status to F-1.
Optional Practical Training
M-1 students are eligible for a relatively short amount of Optional Practical Training: one month for every four months of study (as laid out in Title 8, Code of Federal Regulations). While an F-1 student can simply file an I-765 Application for Employment Authorization, an M-1 student must also file an I-539 to extend status. The applications are adjudicated at various USCIS service centers around the country, which sometimes have varying interpretations of the regulations.