Sure it's insulting.
I would say it's borderline defamation, and I would not want to test it in court. Not because it's opinion (you're not presenting your opinion: you're not saying "I think these are the professors whom students think are corrupt") but because the fact you're stating is a statistical figure. Given the potentially devastating consequences to a professor of being labelled corrupt, you might want to think long and hard about how you're going to back up your statement. What "students" think those profs are corrupt? The way you have it worded could easily be construed as "all students". Arguably, that is a gross overstatement. What percentage of the student vote does a prof need to get to be considered corrupt? How did you draw that line? How do you intend to prove "students think they are corrupt" - that is, how reliable are your statistical methods? How do you define "corrupt"?