Final answer:
During Harry Potter's OWL exams, the Weasleys set off fireworks in the 'History of Magic' exam, symbolizing their resistance against Dolores Umbridge's control of Hogwarts.
Step-by-step explanation:
The exam that Harry Potter is taking when the Weasleys let off fireworks in the exam room is the Ordinary Wizarding Level (OWL) exams at Hogwarts. Specifically, the incident you're referring to occurs during Harry's History of Magic exam, which is rudely interrupted by Fred and George Weasley's spectacular fireworks display. This event is described in detail in the fifth book of the series, 'Harry Potter and the Order of the Phoenix', and serves as a symbol of rebellion against the oppressive regime of Dolores Umbridge, who was overseeing Hogwarts at that time.