In 'A Harvest of Friends', there is a window visible behind Dr Baker, Mr Hanson and Ma while they are talking with Pa who is in bed with broken ribs. In 'Country Girls', Ma lifts a chest out from under the bed. The window has disappeared.
When Mary and Laura arrive for their new school, they walk to the front to see Miss Beadle. Laura does not have the lunch pail. After talking to Miss Beadle and walking to their seat, Laura has the lunch pail.
In 'A Harvest of Friends', while Dr Baker, Mr Hanson and Ma are talking to Pa who is in bed with broken ribs, the ladder to the loft is not there. In 'Country Girls', while Ma is getting the school books for the girls, the ladder is clearly visible.
Laura and Miss Beedle are reading a book called "Dicky Bird Land." Historically this would be 1870s. Dicky Bird Land was published in 1900.
Toward the end of the episode, Laura and her mother walk back into the school house to show Eva Beadle Laura's essay. Ms. Beadle finishes erasing the chalk board. The camera shows a close up of the essay then goes back to a full shot of the characters in the school room. On the chalk board the word 'Welcome' has appeared, which Ms. Beadle then erases.
In fact, in the first shot, Miss Beadle does NOT finish erasing the blackboard; The word 'Welcome' is still written.