The Text Dossier
The students making those claims were not, personally, in attendance. Rather, they compiled a series of text messages from classmates into a "dossier," which they believe proves that the secret dinner parties occurred. However, the text exchange between one aggrieved student and the one who visited the professor's home offers no proof of the supposed revelry taking place - just one-sided demands from the former to admit that there were judges there, which the latter repeatedly denies.
Not exactly the solid proof a law student should think would hold up in court...