In Harry Potter and the Chamber of Secrets, Harry and Ron find out that the Hogwarts caretaker, Argus Filch, is a Squib.
"And what on earth's a Squib?" said Harry.
To his surprise, Ron stifled a snigger.
"Well – it's not funny really – but as it's Filch ..." he said. "A Squib is someone who was born into a wizarding family but hasn't got any magic powers. Kind of the opposite of Muggle-born wizards, but Squibs are quite unusual. If Filch's trying to learn magic from a Kwikspell course, I reckon he must be a Squib. It would explain a lot. Like why he hates students so much." Ron gave a satisfied smile. "He's bitter."
— Harry Potter and the Chamber of Secrets by J.K. Rowling, Chapter 9, "The Writing on the Wall"
Filch reluctantly talks about it in front of a number of teachers, such as Snape and Dumbledore.
"[...] He found—in my office—he knows I'm a—I'm a—" Filch's face worked horribly. "He knows I'm a Squib!" he finished.
"I never touched Mrs. Norris!" Harry said loudly, uncomfortably aware of everyone looking at him, including all the Lockharts on the walls. "And I don't even know what a Squib is."
"Rubbish!" snarled Filch. "He saw my Kwikspell letter!"
— Harry Potter and the Chamber of Secrets by J.K. Rowling, Chapter 9, "The Writing on the Wall"
Yet Umbridge, who had a Squib brother herself, did not seem to know he was one.
"Don't stun them, Filch!" shouted Umbridge angrily, for all the world as though it had been his incantation.
"Right you are, Headmistress!" wheezed Filch, who as a Squib could no more have Stunned the fireworks than swallowed them.
— Harry Potter and the Order of the Phoenix by J.K. Rowling, Chapter 28, "Snape's Worst Memory"
Umbridge had a Squib brother and a Muggle mother, with whom she broke all ties, so it doesn't seem likely she would be considerate enough to cover for a Squib — it's likely she just didn't know.
So who all knew that Filch was a Squib?