It's almost universal that a student can call witnesses in a university disciplinary hearing. A concern would be schools often make evidentiary determinations in terms of who they feel would be an appropriate witness or who would have relevant information. They could overstep their bounds in that regard. Having an experienced attorney advisor helping push back as necessary is sometimes critical, but it is an almost universal right where a student can call witnesses either during the investigation or at a hearing.
Another concern, however, is that if you offer, say, well, these three people have relevant information, we would like these people interviewed. Sometimes schools, as ridiculous as it may be, you find out later we interviewed one person. Well, what happened to the other two people? Well, we figured we had enough information. Well, that's not the way to ensure the prospect of success. That's why an experienced attorney advisor needs to be in your corner throughout the process to make sure that your rights are heard, so that a school does not rush to judgment and overlook your rights and interests. They need to be involved throughout the process.