Iain Turnbull, headteacher of Cannock Chase High School, said in a statement that the school had made a "mistake" in handling concerns about uniform standards among a group of pupils.
Six teenagers, all believed to be aged about 14 years old, were rebuked about the length or tightness of their skirts in front of fellow pupils.
This led to other pupils choosing to go out of their classrooms in protest the following day.
On Monday all pupils were said to be back in their classes once again.
Mr Turnbull said: "At Cannock Chase High School we have the highest expectations in relation to academic achievement, personal development and equality of opportunity.
"This extends to our expectations in relation to behaviour and uniform.
"On Thursday, June 10, while maintaining these expectations, we made a mistake in how we addressed uniform concerns in a year group and we apologise for this.
"This is not representative of the school’s practice and was an isolated, individual error of judgement.
"On Friday, for a short part of the early morning, a small number of our students wished to voice their feelings regarding the matter as part of a peaceful on-site protest.
"We are meeting with these students and their parents or guardians to ensure we work constructively together.
"As of today all students are now in lessons and focused on their learning.”