The gripping X-Men: Evolution animated series adopts a slightly different perspective on the X-Men as compared to the conventional lore. As discussed in this post, the series tinkered with "the age of X-Men characters, making most of them teenagers" and enrolled in the Bayville High School (with Mystique originally disguised as the Principal of the school, in Season 1).
The first episode of Season 2 is epoch making, as the Brotherhood of Mutants decide to "come out", revealing their mutations to the public. During the process of neutralizing their offense and saving general public from the destruction they unleash, the X-Men teenagers end up revealing themselves too, much against the desire of Prof. X who has been believing right through that the X-Men better stay disguised till the society at large is ready to accept mutants. With no choice left to prevent the mutant unveiling, Prof. X decides to take the extreme step of using his powers to erase the event-memory of everyone present at the venue:
As we see at 18:44 in the above clip, Prof. X successfully erases the memories of all-but-one of witnesses, finally attempting to do the same to the new principal of the school, Edward Kelly. Upon attempting the same with him, the Professor encounters some sort of resistance, or a psionic burst, as a consequence of which he passes out.
Clearly, it must have been a powerful mutant disguised as Principal Kelly, with psychic powers comparable to Prof. X, or perhaps even superior. However, apart from Prof. X, the only psychic mutants revealed in the series who could fit this description, were the teenaged Jean Grey (who was both an ally, as well as affected party here), Lucas and Mesmero (who showed up only later in the series). (Note: Apocalypse still hadn't woken up until this point in the series.)
Until the end of the fourth season, after which the series stopped, this arc of the story was unexplored. All we got to know of Principal Kelly's character description can be succinctly summarized as:
Edward Kelly was the second principal of Bayville High (the first, Ms. Darkholme, aka Mystique, had vanished) and carried suspicions about the mutant teens which became full-blown prejudice when they were publicly revealed, causing him to make reforms at the high school. Later, he ran for mayor of Bayville competing with the falsely heroic Brotherhood for media attention, a nod to his career as a politician in the comics.
Which powerful mutant was disguised as Principal Kelly of the Bayville High School?