The school took action after the app went viral within one day, with hundreds of students downloading it to their smartphones.
It allows students to post comment anonymously, like a giant message board. Only those in a 16km radius can see what's happening, and are able to either "up vote" or "down vote" comments. If a comment, or "yak" gets many upvotes, it gets a high score, rewarding the user with "yakarma". Messages can also be commented on, starting a thread of conversation.
At Diocesan, students noticed nasty comments about both students inside the school and other schools in the area.
Ms McRae said when the school investigated, it found the app had caused issues in schools overseas as well.
Its anonymity made it a perfect app for bullying, she said.
What is Yik Yak?
An anoymous social media app that allows people to make posts and comment on others' posts within a 16km radius.
Posts can be "up voted" or "down voted" giving users a success rating
It was released in 2013 and has received negative feedback in the United States, where it was criticised for contributing to bullying and harassment in schools