{Real world logic} The grade scam is still career ending, so it is sufficient blackmail. If it was made pubic she wouldn't be able to get another teaching position. Granted that could happen if her publicly humiliating the MC in the beginning was known as well.
{Porn logic} It could have been an embarrassing pic and it still would have been enough.
{Porn logic} It could have been an embarrassing pic and it still would have been enough.