If you want to compliment a teacher...you want to say: Exceptional teachers are a blessing and I am truly blessed with you.

It's not grammatically correct. "Exceptional teachers" is plural. So you shouldn't say that you are blessed with "it." You could say that you are "truly blessed with one," meaning one exceptional teacher. Or you could say that you are truly blessed with "them," meaning that you're blessed with multiple good teachers. NOTE: The way it's written, you could be talking to someone who provided you with a teacher (or with teachers). If you're talking to the actual teacher, then I'd write: "Exceptional teachers are a blessing and I am truly blessed by having you."