I worked as a strength and conditioning coach for wheelchair athletes for a while. One of the first things I did during my training for that was actually do some of their workouts in a wheelchair, even though I can walk, just to see what it's like and understand their perspective better. It allowed me to empathise with my athletes better, as well as make sure that the exercises I was programming for them were actually feasible to perform in a wheelchair. This is the exact same type of situation. The intent behind and the context surrounding an action like this means everything. Are you imitating a stutter because you think it's funny or you deliberately want to humiliate someone who stutters? Or are you imitating a stutter so that you can understand it better and help people who have it? Honestly it says a lot more about your classmates that they got offended on other's behalf and cared more about the possibility of feeling some righteous indignation than they did becoming better medical professionals