Six teenage girls stood in front of 300 classmates while they were told how short and tight their skirts were.
Students protested outside the school on Friday after the incident during a gathering.
The mother of one of the 14-year-olds said her daughter felt humiliated.
Joanna Stockle, who works in recruiting, has since received an apology and an assurance that the matter will be investigated.
Daughter Molly said, “I felt humiliated, I just didn’t understand what was going on.”
Mother Joanna said Molly wore the same skirt throughout her time in grade 9. She said, “Molly is gutted, she couldn’t catch her breath before crying.”
Ms. Stockle said: “Molly wears tights to school, she is very classy. She believes that there is a unified policy for good reason.
“The headmaster is very good and very understanding. It was none of his business, but, as he said, his name is on the board. “
A school statement said: “At Cannock Chase High School, we have the highest expectations for academic achievement, personal development and equity.
“This extends to our expectations for behavior and uniform.
“Yesterday (Thursday) we maintained these expectations, but made a mistake in addressing uniform issues in an annual group, and we apologize for that.
“This is not representative of the school’s practice and was an isolated, individual misjudgment.
“Today, for a short part of the early morning, some of our students wanted to express their feelings on the subject as part of a peaceful local protest.
“We meet with these students and their parents / guardians to ensure constructive cooperation.”