"So this would be, at a guess, 11 years ago?"

I was in 4th grade back in the early 80's.

"Let's try to expand that description a little bit. Were the books paperback or hardcover? (If they were boxed, probably paperback, but you never know.) If paperback, do you remember if they were stapled in the center or bound with glue, like a standard paperback? (This will help to determine the size of the books.) Do you remember if they were more square or more rectangular? Did I understand you correctly, that there was only one story per book?"

Well "book" is a little of a stretch. They were short stories several pages in length with rendered pictures too. I seem to recall them being stapled but I'm not 100%. And yes, there was only one story in each.

"Were these books used only in school, or did you get to take them home? Were they ever made available to buy, that you remember? Were the questions objective (fill in the blank, matching, multiple choice) or were they short essays? Did they come in colors that indicated different reading levels, and if so, were you expected to read the colors in a particular order?"

Yes, they could only be read during certain times in class. No, we couldn't buy them. I can't recall the nature of the questions. There may have been an indicator similar to that, but I'm not sure. It does seem like the stories were numbered to be read in a certain order.

"The short answer to the first question is, no, not yet. :) It sounds like a set of reading curricular units, though, and might have been produced by Macmillan, McGraw-Hill, or some of the other big school text publishers specifically for classroom use."

Yes, they were sets of reading curricular units

One of the stories I really remember was of a family vacationing on another world. On that world were giant ancient statues left over by a long vanished alien race. The family had been there a few years before and the statues weren't in the same location or in the same positions. Turns out they WERE the aliens. The statues were alive and moved at a very slow rate compared to us. Just looking at them you couldn't tell, but over the course of years you could see them move. It was a very interesting story.