Duffy uses a simile here. Just as an uneasy mistake would be rubbed out but still make a person feel uncomfortable as it never really goes away so too the names of the serial killers, which would frighten children at the time, may not be heard in the class, but they remain an uncomfortable presence in their minds. However, when in Mrs Tilscher’s class, the children can forget for a while as she provides a safe and comforting presence.