The only other option I can think of, would be no leggins at all, which we saw in the stage play.
It worked there (just barely though) but only because it was worn by a 20-some year old, and not an actual minor, as in the TV show.
Ultimately I think it was a mutual decision made by LTE and Nickelodeon, in order to appeal to the right audience.
If only other tv-shows would have turned down the sexappeal from their lead characters as well, I probably wouldn't have spent my childhood madly in love with Totally Spies, Kim Possible, Braceface etc. etc.
inb4 "if it's anime it's okay"... >.<