"We always have some fun on this day, and there's prizegiving tonight."
Mr Shepherd recalls other pranks, including one year when a teacher's car was boxed in with cardboard boxes.
"We have some ground rules, such as not hurting anyone's feelings, or [damage to] property."
A Year-13 student, speaking anonymously to the Times-Age, said she was not involved but thought the sign was "really funny".
The "sale", advertised as a means to buy a school van, was a nice touch as the school was genuinely fundraising for a van, she said.
A member of staff, who tipped off the Times-Age on the promise of anonymity, said the sign did confuse some students.
"'Don't we come back next year?', some asked.
"People told them, look at it, it's a joke."
She said people thought it was "brilliant".
In previous years students have covered cars with cling film, filled up teachers' offices with balloons, and on one occasion, placed every school clock into a classroom used by a teacher who didn't like clocks.
Mr Shepherd said the day had a tinge of sadness in farewelling students after five years.
"It's their last formal day, and they spend the day doing pranks, and dress the auditorium up. It's a nice feeling."
