Morrie Tobin, the Yale dad who allegedly tipped off federal authorities to what they say is the largest college-admissions cheating scam they’ve ever prosecuted, wasn’t just an informant. He was also among the parents who participated in the scheme, according to a person familiar with the investigation.

Mr. Tobin, a Los Angeles financier, led authorities to the alleged college-admissions scheme, The Wall Street Journal reported last week, citing people familiar with the investigation. But what hasn’t previously been reported...