AOPA website says:
According to INS regulations, Title 8 of the Code of Federal Regulations, Section 214.2(b)(7), any alien who is permitted to enter the United States on a B-1 or B-2 visitor's status visa is prohibited from enrolling in a course of study. Such an alien must an either obtain an F-1 or M-1 non-immigrant visa from a consular officer abroad and seek readmission to the United States, or apply for and obtain a change of status under section 248 of the Act and 8 CFR part 248. The alien may not enroll in the course of study until the Service has admitted the alien as an F-1 or M-1 nonimmigrant or has approved the alien's application.