The technician, who has since retired, also lost parts of three fingers while preparing an explosive for a fireworks demonstration for a class at Bristol Cathedral Choir School.
It was revealed that explosive substances were regularly handled and prepared at the school, with gunpowder and flash powder kept in its chemistry storeroom.
The technician spent 12 days in hospital as a result of the accident, which the Health and Safety Executive (HSE) said could have been avoided if the school had clear management arrangements to control and review the risks posed by the chemicals used in its teaching activities.
The school was fined £26,000 (£8,000 for the Section 2 offence and £18,000 for the Section 3 offence) and ordered to pay £12,176 costs.
