(Hey. Did you know there is such a thing as a “drive-thru funeral home,” and now there’s a “drive-thru church”? Whew. But since this is a language blog, let’s just focus on the use of thru.)
Thru is an informal, simplified spelling of through. It is akin to spelling night as nite. Neither thru or nite is appropriate in formal writing, but, let’s face it, both are useful in texting.
The word thru appears acceptable only as an abbreviation, in which case it can be used in the heading segment of a memo (together with the other parts of the heading segment such as TO, FROM, SUBJECT), but it is not advisable to use it in the body of memos, emails, letters and reports. Or a love letter to an English teacher.