Many of the adults that are rarely referred to on a first-name basis seem to have used multiple names over the years.
Emil Ratburn became Nigel Ratburn by the fifth season, Herb Haney became Francis Haney at some point - and of course Sarah MacGrady became Leah MacGrady by the cancer episode.

It was my impression that "Leah" in the Mystery Envelope episode was supposed to be someone else.