You can hear Mr. Lawrence call Martin Schlegel to the office. Martin was a character in the Kids of Degrassi Street series, but has not actually appeared on DJH.
Regarding the Canadian vs. US $20 bills, two different productions were made of Degrassi: one to air in Canada and one to air in the USA. Cultural references were adjusted slightly, that's all. I've seen both versions of this episode. The one I borrowed from the Toronto Public Library (Canadian) shows a 1970/1978 issue Canadian $20 bill, whereas the one I got from WGBH shows the classic US $20 greenback.
I seem to remember those being Canadian 20's.
In response to the above goof, I listened to it several times and I think she said "kiddo", but it did sound an awful lot like Caitlin. I'm just saying she said kiddo, because she refers to Melanie as kiddo several times in the episode.
In this episode Melanie's mom calls her Caitlin instead of Melanie when she comes home from school. Also, although the show is set in Canada, Melanie's mom apparently uses American bills instead of the more popular Canadian dollars.
Kathleen still hates Melanie.