The student, who wished to remain anonymous, said the school raised the commission rate from 20% to 25% last week, less than two weeks before the exhibition, which opens today.
Given that students paid course fees and had to cover the cost of materials used in their artwork, they felt the level of commission was "ridiculous", the student said.
It meant students would need to raise the price of their works to cover the cost of materials, reducing the chances they would sell.
"It's really stressful.
"This is supposed to be about celebrating our work, but we now have to stress out about how much money we are actually going to get out of it and ...[whether] we are actually going to cover our costs."
The school had also failed to tell them why the rate was raised to 25% this year.
It was not the fact students had to pay commission, but the lackof communication and the fact the rate was so high that frustrated students.
"If [the school] had explained why, then the commission would be easier to accept."
Otago Polytechnic director of communications Mike Waddell said art students always had to pay commission at the end-of-year exhibition.
"The reason for the increase is that the commission has been at 20% for the last 10 years and, as you can imagine, costs have gone up during this time, hence the 5% increase," Mr Waddell said.
The "reality was" the art school subsidised the exhibition, which made a loss.
It was common for artists to be charged 40% or more for artworks sold at private galleries and to have to pay for the hire of the venue, he said.
Fifty graduating students from the school will show their work at this year's exhibition, SITE 2012, at the Dunedin School of Art in Riego St.
The exhibition is open to the public today from 10am-4pm and next Monday to Thursday from noon to 4pm.