"The Book and the Beast", a short story by Robert Arthur; first published in Weird Tales, March 1943, available at the Internet Archive; reprinted in Arthur's collection Ghosts and More Ghosts. Also released under the title of "Mr. Dexter's Dragon". Perhaps you read a bowdlerized version; the original Weird Tales version does not seem wholly suitable for elementary school.
The story centers around a curious book that passes from owner to owner throughout the story. I'm fairly certain the first owner of the book picks it up in a curious little bookshop of some sort.
Waldo Murchison found the book in the most prosaic of places — a second-hand shop. Not even a good second-hand shop. Just a dingy hole in the wall on Canal Street, east of Broadway, a region as commonplace as Manhattan has to offer. It was a shop devoted chiefly to second-hand luggage and old clothes of the most depressing appearance.
[. . . .]
"Hmm," Waldo Murchison commented, in a non-committal manner, and turning
a lackluster eye upon the proprietor, shook the book slightly. "What is it?" he sighed.
It was, he gathered from the instant reply, a volume of the utmost rarity, the personal diary of a European nobleman of note, an intimate friend of Napoleon's. It had been found in a suitcase bought by the proprietor himself at a sale of unclaimed luggage from the various hotels. It had belonged to a European gentleman who had been so crass as to run out on his
hotel bill — at least, he had vanished from his room and never been seen again — and so had come into the proprietor's possession with the utmost legality.
In any case, the most distinguishing feature of the book is an vividly illustrated page with a painting of a dragon.
Murchison describes his find in a letter to his friend McKenzie Muir:
The most noteworthy item in the volume, though, is an inset parchment containing the brilliantly colored picture of a dragon. The monster has green scales, long blue claws, blue fangs in a crimson mouth, a scarlet tail, and scarlet filaments or antennae dangling from its head and spine like seaweed. Its eyes are bright yellow shot through with scarlet, and gleam from its head with an almost living brilliance.
The dragon seems to be squatting upon a tiled floor of stone, looking directly at you, jaws slightly agape, and a ravenous expression plain upon its features. Its scaled flanks are lean and sunken. Its bones show through
everywhere. A leaner, hungrier, more sinister monster I have never seen pictured. I have, accordingly, decided to nickname it Cassius.
The dragon looks quite lifelike, and is sitting on a number of skulls. I'm not certain of the number of skulls, but let's use the variable "X" to start.
X = 13. Murchison's letter to Muir continues:
Behind the dragon, partially obscured, is a pile of bones — a pleasantly gruesome touch. For they are human bones. I have examined the picture through the glass, and there are visible thirteen human skulls, so skillfully done by the artist that under magnification every detail of them is accurate even to the discoloration showing some to be older than the others. Mingled with them are a mass of other bones and shreds of cloth; the whole is startling and almost upsetting in its vivid accuracy.
The original purchaser of the book disappears mysteriously one night.
When the police arrived, they made slight headway in fathoming the mystery of Waldo Murchison's disappearance. He was just gone, with nothing to show for his going save a slight disturbance of his study. Some books had been
knocked off his desk, as if swept off by a careless arm, and Mr. Murchison's glasses had fallen to the floor and broken.
After a time, the book is purchased by another individual,
Rather, Murchison's friend Muir cons the police into letting him take the book, which he claims to have loaned to Murchison.
who also comments on the vivid drawing of the dragon. The reader notes that the dragon is sitting on X+1 skulls.
Then, seeing the volume on his desk, he picked it up for one more look and found himself fascinated again, as Waldo Murchison had been, by the repulsive little dragon.
After a moment he reached for a glass to study it more carefully. And doing so, he snorted, for he perceived that Murchison had been guilty of an inaccuracy. A distinct inaccuracy in describing the beast.
"'Lean and hungry'!" he sniffed aloud. "'Bones showing through everywhere.' Gross overstatement. The beastie is not fat, to be sure, but his bones don't show through. And though one might say his expression was hungry, I'd not call it ravenous. There's even a bit of bulge to the belly, which is not an indication of starvation. And — " Muir peered more closely through the glass — "there are fourteen skulls, not thirteen, in the heap behind the
beastie. Ha! It’s not like old Murchison to be so careless. No doubt he did wander off somewhere with amnesia. Must have been slipping in his mind to make so many mistakes!"
I think one of the later owners mentions how well-fed the dragon in the picture seems to be.
As he handled the volume, which the police had glanced at, closed, and put
aside, it fell open in his hands at the picture of a small dragon. Johnson gazed at it for a moment with passing interest.
"Jolly fat little beast," he commented to Dora, the maid, who was locking the windows. "Got a grin on him from ear to ear." Then, closing the book and putting it away, they left the study to gather dust.
I read it as part of a reading assignment in elementary school, which would put it early 80s.
There is a passage that may have been edited in the version you read in elementary school:
The whole thing is written in a hash of Latin, French, and Italian. This I take to have been an additional precaution against unauthorized use, since only a very well-educated person could possibly have read it. It will take considerable digging to make the necessary translations, but I have already partially deciphered two of the conjurations. One is simply called, "To Be Invisible." The other, "To Bring Three Beautiful Females to Your Room After Dark."