"During the past week I have witnessed several pranks taking place and ones involving eggs, flour or anything but water were all done by other schools. I feel it is unfair that our year got our last day taken of us because other schools took the pranks too far."
Claire McNab said she and others had taken responsibility for trying to waterbomb students at another school.
"The only other pranks we did were filling teachers offices up with balloons and blocking the stairwells, which are all pretty harmless."
Another student said egging, flour bombing and plastic wrapping of cars was done by the boys from a nearby school to the cars of Westlake Girls students.
Another teenager said the worst prank Westlake Girls carried out was "a harmless mission where approximately 40-50 girls dressed in black attempted to enter the boys school during their lunchtime with water bombs and water pistols but were denied entry upon arrival".
The students were told by the Year 13 dean "today is your final day" in an attempt to prevent further pranks from being carried out yesterday.
But, principal Alison Gernhoefer, who is retiring at the end of this year, said around half of the Year 13 students attended school yesterday - about the same number as usual on the last day of school. She said staff had "no idea" who was responsible for egging cars but said senior staff were concerned about "things escalating".