Is Yik Yak 100% anonymous?

Since Yik Yak is an electronic medium, the answer is that tracing the posting back to you is probably possible. Depending on what you post, the school might not care enough to bother, but if your post is threatening, bullying, or otherwise offensive, the school might either conduct an investigation or turn the matter over to authorities. The smart policy is never to post anything electronically which you would not want to have identified as yours.