Enrollment for foreign students in the USA is handled by the Student and Exchange Visitor Program (SEVP), a division of the Department of Homeland Security.
Students between the ages of 14 and 79 must also undergo an interview with the embassy consular section as a condition of receiving a visa. Student visas remain valid for the entire program of study as long as you remain enrolled full-time. You are allowed to return to your home country for brief visits without reapplying for a visa.
Foreign students enrolled in undergraduate or graduate programs are allowed to work up to 20 hours per week on campus during the first year of study. After the first year, you may apply to work up to 20 hours per week off campus in a position directly related to your program of study during the academic year. Under certain circumstances (especially pertaining to graduate students), you may also be permitted to work full-time during the summer months.
Students who hold a F-1 visa and who enroll in public secondary school are limited to a maximum of 12 months of enrollment. In addition, the student, or another individual, must pay the full, unsubsidized cost of the student's education for the entire period of enrollment. Students in other visa categories, and students who hold F-1 visas and enroll in private school are not subject to this restriction.
In most cases, students may enter the United States no more than 120 days before the beginning of their educational programs. Most students must return to their home countries for at least two years after finishing their studies. If you wish to remain in the United States after completing your educational program, you must apply for a waiver using Form I-539.