About 250 pupils at Woodlands Academy, an all-boys' school in Coventry, went on strike over a potential merger with Tile Hill Wood girls' school.
Woodlands has been declared unviable because of a drop in demand for single-sex education.
However, parents and pupils say the merger would go against their wishes.
More than 2,800 people have signed a petition launched to save the school, whose famous sporting alumni include rugby stars Neil Back, Danny Grewcock and Tom Wood.
Former Woodlands pupil Mr Moorcroft - who broke the world record for the 5000m in 1982 - backed the pupils in their protest and said he felt sorry for students and staff.
"It was perfect for me, and I feel really sad that Woodlands might close because it's part of the infrastructure of Coventry," he said.
"I really worry that if something happens to Woodlands then it might happen to other schools."
Kris Roscoe, one of the pupils who started the petition, said teachers were "gutted" about the plans.
"They're worried at the moment and they don't know how to conduct themselves," he said.
Parent Sarah Illsley, who found out on Facebook, said people needed to be able to choose to have single-sex education, and criticised the lack of information.
"We just  feel as if we've been hung out to dry," she said.
The BBC has been unable to contact the school for a response.
Former Commonwealth champion David Moorcroft has backed a protest by pupils against plans to close down their school and merge it with another.