Your idea of sending different pieces of information to different suspects and observing the outcome is a classic approach known as a "canary trap." It's been used in counter-intelligence to identify leaks. However, you should consider the ethical implications and potential legal issues related to this method.

Another approach, more technical, could be to analyze the headers of the defamatory emails. Email headers contain metadata about the email, including the originating IP address. This could potentially help identify the sender.

Lastly, involve your school's legal team or local law enforcement, as they might have more resources and expertise to handle such a situation.