St. Joseph's High School students were told the snug leggings most recently made trendy by the lululemon clothing chain needed either a skirt to cover the behind or a long, untucked shirt, CTV News reported.
Ottawa Catholic School Board spokeswoman told the Toronto Star the measure was only enforcing its existing dress code and wasn't targeting lululemon.
"There's a misconception out there that we've banned a specific brand," she said. "Any form of defiant behavior in school would have consequences."
An unidentified student told CTV warnings had been issued saying anyone wearing the yoga pants would get a detention on the first offense, a suspension the second time and expulsion for the third.
Another girl told the broadcaster the rule was unfair.
"Guys at our school can wear their pants around their ankles and they don't get in trouble," the girl said. "Girls wear something a little too tight or a little too revealing and the whole world comes crashing down."