For a map, see
https://en.wikipedia.org/wiki/Perth_Modern_School#/map/0.
First you need to find Administration near the corner Roberts Road
with Thomas Street.
Then AU20 (Andrews building Upper level room 20) is in the same building.
Go past reception, i.e. go north, then turn right and
go up a flight of stairs.
And keep going.
Eventually, you should find the room on your left.