Food Safety deputy director-general Vincent Arbuckle says it was found in a meal at Kelston Boys' High School on May 12.
He said the larva had been sent away for testing and the results were expected back next week.
The tests should confirm what type of insect it is and help determine how it might have got into the food.
He said although officials were looking into the complaint, at this stage there was no evidence there was a wider problem with the School Lunch Collective which provides the meals.
The collective has been approached for comment.
The lunch scheme was plagued by problems in term one, with criticism of late, inedible, repetitive or nutritionally lacking lunches, and even a case of a lunch containing melted plastic.
The Auditor-General's office has announced plans for an inquiry into the programme.
