"Mid-exam students would ask to use the toilet only to actually use their phones to take a photo of their gluteus maximus in the mirror and then memorise what had been written on it before heading back to the exam room.
"In the past Avondale has also seen students write answers on genitalia, upper thighs and even small equations on nipples."
As a result the letter said a full "butt-cheek and Body inspection" would be conducted prior to each exam session.
The letter also claimed those who failed to comply would not be granted entry.
Mr Lewis believed ten copies had been made and were circulated around a number of Year 13 classes.
He said the letters have since been taken down.
"We are currently dealing with the matter and are also talking to the parents of those responsible."
A caregiver of one of the school's students, who did not want to be named, said she'd initially fallen for the prank.
"I thought it's either gone too far and you've got to be kidding me, or it's hilarious."
She said when she rang the school they wouldn't confirm to her whether it was real or not.
"They said that there had been a few complaints and they were looking into it."
However, Mr Lewis denied the prank had sparked any complaints.
Yesterday afternoon, the Herald spoke to two Avondale College Year 13 students, who confirmed the notes had been placed on classroom windows that morning.
"When I walked into form class, my teacher just had it, kind of read it out. It was like a funny joke thing," one of the students said.
He thought it was a prank by Year 13 leavers and possibly even the prefects.
"Everyone kind of knows it's a joke, it's more of just speculation on who would have done it.
"It might have been the prefects who did it ... They were planning a prank but yeah, we don't know if that was the prank."
The two students were unsure if anyone had been apprehended.
