The name of the book is 'Ludlow's Mill' by R.R. Walters. A grateful shout-out to Will Erickson--'Paperbacks from Hell' author--and his blog Too Much Horror Fiction. He circulated my question among his readers and one of them came up with the title and author, and even the front and back covers. Happy to have found this again.
This is "The Tale of the Hatchling", episode 12 of the second season of the original Are You Afraid of the Dark? show.
.... A vehicle pulls through the gates of a boarding school called "The Black Brook School" supposedly one of the best boarding schools around. Mr. and Mrs. Wilson who had to go away on business for six months thought it ...
This is the Arthur Porges short story “The Mirror.”
It is as the poster described with an entity coming through the mirror world reflected in an antique mirror. The monster, if I recall correctly, is described as spider like.
Found it! It's in Forbidden Worlds #129 (ACG-1965). But not quite as I thought. The Stack Exchange Effect (I post; it's found) once more. And only 11 days.
An anthology series featuring tales of fantasy and science fiction adventure from legendary early independent comics publisher ACG. Magicman battles aliens who have taken over the bodies of a team of ...