"The girls organised it all and one of our students, as runner-up in the National Race Unity competition, read her wonderful speech to the Prime Minster. It was so special.
"The Prime Minster gave an address about some of the problems faced by youth today and talked a lot about mental health issues facing students. The whole thing was so successful and absolutely wonderful.
"So it's devastating that one student's silly, silly actions have marred what was a hugely successful event for the 1399 other students and staff."
She said the students were gutted and upset at what had happened, and were united in their opposition to the incident.
"They are absolutely devastated that it could be undermined by the actions of one student."
Ms Cooper hoped the incident did not reflect badly on the school as a whole and its students.