Those who were after the reference regarding B1/B2 visas.
This is the extract from 8 CFR § 214.2 which does not allow any studying on a B1/B2 visa.
(7) Enrollment in a course of study prohibited. An alien who is admitted as, or changes status to, a B–1 or B–2 nonimmigrant on or after April 12, 2002, or who files a request to extend the period of authorized stay in B–1 or B–2 nonimmigrant status on or after such date, violates the conditions of his or her B–1 or B–2 status if the alien enrolls in a course of study. Such an alien who desires to enroll in a course of study must either obtain an F–1 or M–1 nonimmigrant 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 under part 248 of this chapter and changed the alien's status to that of an F–1 or M–1 nonimmigrant.