New England Conservatory officials are now in the stressful waiting period to see if any of its thousands of young students, past or present, come forward with complaints about a longtime school videographer who, they said, hid his past as a registered sex offender and whose crimes nearly 20 years ago included secretly taping himself having sex with teenage boys. Meanwhile, Karen Schwartzman, a spokeswoman for the internationally-acclaimed conservatory, acknowledged the school could have discovered this information about Peter E. Benjamin, 68, had it followed its own policy, instituted in November 2010, of running criminal background checks of all vendors, not just faculty, staff and volunteers.