"'Nothing Happens on the Moon'" (sometimes printed with quotation marks in the title, sometimes without), a short story by Paul Ernst, first published in Astounding Science-Fiction, February 1939, available at the Internet Archive. Does any of these covers look familiar? If you read it in school, it might have been in the 1996 anthology Science Fiction, Science Fact, and You from Amsco School Publications. When were you in middle school?
The protagonist is all alone in a station on the Moon:
Clow Hartigan was a big young man with sand-red hair and slightly bitter blue eyes. He was representative of the type Spaceways sent to such isolated emergency landing stations as the Moon.
There were half a dozen such emergency landing domes, visited only by supply ships, but ready in case some passenger liner was crippled by a meteor or by mechanical trouble. The two worst on the Spaceways list were the insulated hell on Mercury, and this great, lonely hangar on the Moon. To them Spaceways sent the pick of their probation executives. Big men. Powerful men. Young men. (Also men who were unlucky enough not to have an old family friend or an uncle on the board of directors who could swing a soft berth for them.) Spaceways did not keep them there long. Men killed themselves, or went mad and began inconsiderately smashing expensive equipment, after too long a dose of such loneliness as that of the Moon.
[. . . .]
United Spaceways had been petitioned more than once to send two men instead of one to manage each outlying field; but Spaceways was an efficient corporation with no desire to pay two men where one could handle the job.
He makes monthly, not daily, reports to Earth. This is from the end of the story, after the encounter with the space monster:
"Hartigan talking. Monthly report."
"All right, Hartigan."
A hurried, fretful voice. Come on, Moon; report that, as always, nothing has happened.
"Lunar conditions are the same," said Hartigan. "No ships have put in, or have reported themselves as being in distress. The hangar is in good shape, with no leaks."
"Right," said Stacey, in the voice of a busy man. "Supplies?"
"You might send up a blonde."
"Be serious, please. Supplies?"
"I need some new power bulbs."
"I'll send them on the next ship. Nothing irregular to report?"
[. . . .]
"Nothing irregular to report," Hartigan said steadily.
Here, in mid-story, Hartigan thinks over the chain of events involving the space monster:
He sat down to try to think the thing out.
"A smooth, round meteor falls. It looks like an egg, though it seems to be of metallic rock. As it cools, it gets lighter in color, till finally it disappears. With a loud bang, it bursts apart, and afterward I hear a sound like scurrying feet. I drop the pieces of the shell to go toward the sound, and then I hear another sound, as if something were macerating and gulping down the pieces of shell, eating them. I come back and can't find the pieces. I go on with my test of opening and closing the main doors. As the outer door closes, I hear a crunching noise as if a rock were being pulverized, and a high scream like that of an animal in pain. All this would indicate that the meteor was a shell, and that some living thing did come out of it.
"But that is impossible.
"No form of life could live through the crash with which that thing struck the Moon, even though the lava ash did cushion the fall to some extent. No form of life could stand the heat of the meteor's fall and impact. No form of life could eat the rocky, metallic shell. It's utterly impossible!
"Or—is it impossible?"
The invisible space monster is trying to break into the station:
It was useless to radio for help now. Long before a destroyer could get here, the savage, insensate monster outside would have opened a rent in the wall. That would mean Hartigan's death from escaping air in the hangar.
Who would have dreamed that there lived anywhere in the universe, on no matter how far or wild a globe, a creature actually able to damage the massive walls of a Spaceways hangar? He could see himself trying to tell about this.
"An animal big enough to crack a hangar wall? And invisible? Well!"
The monster becomes visible as Hartigan is destroying it:
He got a glimpse of a massive block of a head, eyeless and featureless, that joined with no neck whatever to a barrel of a body. He got a glimpse of five legs, like stone pillars, and of a sixth that was only a stump. ("That's what got caught in the doors a month ago—its leg," he heard himself babbling with insane calmness.) Over ten feet high and twenty feet long, the thing was, a living battering-ram, painted in the air in sputtering, shimmering blue sparks that streamed from its massive bulk in all directions.