Another student, Tom Barrand, said the students planned to enter the school as a group at lunchtime, defying Drumm's order.
"It's more of a sit-in than a protest," he said.
"It's a mature way of getting to say goodbye to everyone, so we can have that last day and hopefully prove the senior leadership team wrong that we can have a really positive afternoon."
A screenshot from the private Facebook event page says: "Most of yall mad as hell that you aint got the last day, so we gonna meet at Rocket Park, walk up to MAGS, do some kind of sit in, chill, show the school that they made a mistake not trusting us."
Tom Barrand said students would be wearing school uniform.
"We'll go and sit in the school quad and sing the school song," he said.
"Someone will go and give the teachers the presents that we have bought for them. We will have a chance to say goodbye."
If they are not allowed in, he said, "We'll just stand out in front and sing the school song, and hopefully we can see our teachers for the last time. That will be really nice because they have had a really big impact on all of us over our five years in the school."
Drumm said the students would have a chance to say goodbye to their teachers on Monday at the Year 13 prizegiving.
But Tom Barrand said not all students would attend the prizegiving and Monday would only be a half-day so there would not be time to see all their teachers.
"We have kind of had the rug pulled out from under us," he said. "Hopefully we can maturely and politely express our disagreement with the course of action our leadership has decided to take."