In my time as a Uni student (1968 - '73), the term "subbie" was considered derogatory, but it was OK to say "sub." As in, "The subs have a field trip next Friday." When I came back years later to work here, it was a bit of a shock that nobody seemed to consider "subbie" an insult anymore. Things change, and it seems to be legitimate Uni jargon at this point.