sure,
but that's the thing
if fewer people ever saw her, it would be even more believable
now with this chapter depiction, the whole auditorium saw her silent casting
which breaks the illusion that no one knows her face
even the other fan girl doesn't recognize her
the anime did it better depicting this part
which I have another continuity error problem in the anime,
which makes this continuity problem in this chapter a double blow to me
which made me ranting this whole nonsense
Again, you're taking the simple premise of "Monica is a shy recluse who avoids public appearances, so her face isn't well-known" and jumping all the way to "Monica's identity as the Silent Witch is a complete mystery and nobody on the planet knows about it", which was never the case. There was a whole panic just a few chapters ago when her old teacher transferred in, who obviously recognises his most successful and famous student. Monica had a moment of uncertainty of whether he'd blow her cover, or if he'd figure out that something was going on and keep quiet. The big giveaway you're talking about has literally already happened, and the pieces are in place for it to come up again later.
Her hidden identity literally only matters inside of this school and around the prince. It's not a national secret, and it's never been presented as a national secret. This is just you not paying attention, imagining a completely different story in your head, and getting upset when the actual manga doesn't play along. Even if the prince was an obsessed Silent Witch fanboy, bordering on stalker, who has exhaustively researched the Silent Witch's origins and appearance, he's no sooner going to assume that the famously magical genius Monica Everett is the same person as his classmate, the meek and bumbling Monica Norton, than you would assume your awkward and clumsy coworker is Superman just because they both have black hair and a good jawline. She's undercover, with a fake name, a fake cover story, and with both the faculty and other schoolmates corroborating her false identity. There's no reason to suspect that she's anyone other than who she says she is.
However, that's only been the case until literally this last chapter. Now that the prince has discovered that the Silent Witch has been at his school, and has actively protected him from an assassination attempt, he's presumably going to start trying to figure out where she could be, and if she's hiding in plain sight. He's likely going to put the dots together for exactly the same reasons as you've described, and come to the conclusion that it's Monica. It's not a plot hole. This is the story that is actively unfolding. Maybe you'd like to join us in reading it, instead of just vaguely glancing at the pictures and coming up with a totally different story in your head.