Huh, I was thought that thou was the honorific form of second person singular in english, but it seems y'all are right. And my teachers were anglophones too, I blame Shakespeare. But that actually makes a ton more sense now that I stop to think about it, since you is both singular and plural. IMO having a distinction between respectful second person singular and casual is not really worth it, all you end up with is a ton of confusion regarding when it is proper to use which, sort of like when you start calling someone by their first name only much more awkward to avoid.

Still it's true that not having a distinct second person singular and plural pronoun is a big problem with English, not sure what could be done about it though.