There were no visible signs that industrial action was being taken at Dinnington, or places like Wales High School, Malby Academy and Aston Academy, with full car parks and no picket lines in sight.
But Mr Blackwood said around 50 of his staff were on strike which was why they had to close to the school to students.
“However, those that were not part of the two unions that called the strike, the NUT and NASUWT, or were support staff, worked a normal day and we took the opportunity to do some training or catch up on tasks,” he said.
“And even those who were officially on strike were still largely working from home because, although they believe in their cause, they also care greatly about the young people they teach and they don’t want to let them down.”
Chris Keates, general secretary of the NASUWT, commented: “Strike action is a last resort but teachers have been left with no choice but to demonstrate their anger and frustration in the face of their genuine concerns being dismissed and trivialised.”