The story might have come from one of a series of anthologies called Out of This World edited by Amabel Williams-Ellis & Mably Owen that were common in schools.
There's about 10 of them. Here's the links with the story lists:
http://www.philsp.com/homeville/ISFAC/t93.htm#A2091
http://www.philsp.com/homeville/isfac/t94.htm
They also compiled one called Worlds Apart, which is also listed in the second link, and references an excerpt from Last and First Men (Stapledon). I wonder if the story I'm looking for is from that work...
I wish I could think of the title and author of the other story in the anthology about the last man being kept in a zoo and being considered the most dangerous animal. I think it was in the same anthology. I have since seen the story, and it is by a well-known author, but I cannot for the life of me think what it is.
There was also possibly an excerpt from Wyndham's Kraken Wakes in the anthology. It's the scene where the monsters first mount the beach and skin people. It was pretty shocking stuff for that age. Odd choice for kids.
On reflection, the Wyndham excerpt, and maybe the story itself was in an anthology called Creatures or Monsters, or something like that, and the theme was 'monsters.'