Horrid Henry, Horrid Headmaster is the eighteenth episode of Series 2.
Henry thinks being Headmaster for the day will be like being King for the Day - only it's not!
Mrs Oddbob is holding a competition for one of the students to become head teacher for the day. Miss Battle-Axe shows some disdain for this, though Mrs Oddbob says that all it is is just some end-of-term fun. Unfortunately for them and most of the school, Henry ends up winning. While Mrs Oddbob attempts to talk to Henry about being a headteacher, as being one isn't as easy as he thinks, Miss Battle-Axe bursts in protesting about someone like him being a headteacher. Henry then retorts by replacing her with Ralph as deputy headteacher.
Henry starts changing rules around the school: he introduces Burgers and Chips into the school canteen (which ends up disgusting Peter), replaces computer studies with computer games, karaoke instead of music lessons and makes "Gonna Be a Rockstar" as the school's new song. Although Sid and Miss Lovely are reluctant to do so, Miss Battle-Axe states that they need to fight back and storms into the headteacher's office to protest against what Henry has been doing. However, he responds to this by making Sid and Miss Lovely run laps and have a cold shower and puts Miss Battle-Axe into detention, having Bert oversee her. Peter attempts to start a rebellion with the rest of the Best Boys Club, only for it to be put down when Ralph finds out and puts Peter into detention. Margaret is put in there as well after she refuses to work for Henry and Ralph.
After detention is over, Miss Battle-Axe, Magaret and Peter decide to fight back. Magaret starts by calling Henry's parents and pretends to be the school, telling them that Henry is falling behind on his work. Ralph ends up not knowing anything about this, but they suggest that Henry is given extra homework which Ralph says in order to get rid of them. Miss Battle-Axe then bursts in as Henry is needed to write the end-of-term students reports. With all of them to do, Henry states about how there will be no holiday for him then. Peter ends up recording this and edits it so that it's like Henry is just saying "No holiday" and shows this to some of the school. Believing that Henry is taking away their holiday, they start to protest against him. He and Ralph successfully hold them off while Henry runs out of the window to Mrs Oddbob to get her her job back. While on their way home, Henry states that although he was headteacher for only a short time, he was able to get some end-of-term reports done. This ends up scaring Peter as Henry states that they were the ones for the two of them and makes him think Henry gave himself a glowing report and Peter got a bad one. Henry happily hands his to Mum, while Peter is reluctant and screams "No!".