Middle grade adventure from 60s-80s about friends and/or siblings who go to school and instead of going home end up in a forest and a strange house?

[Aurora]

I read it in the 90s and it was already ancient with very yellow pages. I asked the librarian for an adventure and she gave me that one. It was the size of those pulpy books, short and thin, almost mass market size. They don’t make middle grade in that size anymore!

[Aurora]

I will add the category in a few hours when I can :)

[bookel]

What country was the story set in?
Do you recall what they did in the strange house?
Why was it strange?
Is it fantasy or realistic?
Could it have been published in the 40s or 50s?

[bookel]

Any by Colin Thiele? https://www.librarything.com/author/t...