Gathering evidence and interviewing witnesses in an academic misconduct case will, of course, depend on the circumstances of the allegations in the case itself. It depends on what a person's being accused of. If, for example, the student's being accused of plagiarism, it may be a response to how the school made the allegations involving plagiarism, if the school used a resource or source to indicate that the student plagiarized by saying that the paper or assignment or whatever the issue was was too similar to other sources. It might require, in some instances, an expert response to such a presentment from the school, in terms of being able to respond to the allegations.
If a student, say, is accused of undue collaboration or collaborating with other students, it may be as straightforward as contacting those other students to present statements that that did not take place. In many cases, there's text messages, emails exchanged, even instant messages, for example, or otherwise on social media. It would be a matter of obtaining and potentially presenting such evidence in a person's favor. Depends on the circumstances and the allegations.
An experienced attorney advisor would be your best ally in terms of being able to help guide and suggest as to what would be best in terms of defending against the academic misconduct allegations. They're most helpful when they're involved from as early as possible in the case.