Putting everything else aside (for good this time!
), a thought struck me while I was
enjoying the shameless fun of Higschool DxD in a state of deep meditation - was there really no way to explain the situation to Koichi at start of the school year, in a roundabout way of sorts ? Tell him to sit down and listen to a story they want to share.
"Once upon a time there was a class. After a classmate who was loved by all died, they kept acting as if he was still there, thus making the mistake of inviting the dead in the class. Each following year a formerly deceased person would "revive" and enter the class, causing death to befall upon students and their families.
All forms of exorcism failed and bodies continued to pile up, until eventually they stumbled upon a solution - doing the opposite of what had invited the dead in the first place. They selected a classmate to be treated as non-existent, thus returning the number of exiting classmates to their original balance. And behold - when they did that, no one died from the "curse" that year. <Insert relevant details that I'm too lazy to type out>. Cool story bro, eh?"
Voila. Wouldn't this essentially accomplish the "cluing Koichi in" part ? Mei's name wouldn't even come up in the story. Neither an explicit statement that they are talking about class 3-3. Neither the fact that they are actually doing the same. Nothing of that actually needs to be mentioned, and should be more than enough for Koichi to understand those parts himself. That should sate his desire to chase down Mei and figure whether she's real or he's having illusions, clue him in that he shouldn't try to interact with her, and should be largely enough to sate his desire to understand what the heck is going on.
All without mentioning Mei's existence, or even the fact their class actually has such "non-existent" person. He would piece together that on his own easy enough. Sounds like a pretty safe roundabout way to explain the situation without actually invoking the sensitive information to me.
Yeah, we now know that Koichi not talking to Mei probably wouldn't have had any effect, because the phenomenon had already started, but still - seems like a fairly obvious way to get the message across while routing around the "must not mention/acknowledge Mei" rule.