Ignorance and laziness have won

[FrodoOne]

Common apostrophe mistakes
1) "It's" is only correct as a contraction of "it is", and an apostrophe would not be correct even in the possessive form.

2) "You're" means "you are", and should not be confused with "your", despite sounding the same.

3) Apostrophes are never used for plurals.

"I didn't find half a dozen people," he said on his website.

Point 3 states "Apostrophes are never used for plurals" (one does not write "boy's" when all that is meant is more than one boy)

However, an apostrophe can be used with plurals.

"The boy's school" refers to the school which "the boy" attends (He does NOT own it!)
A "Boys' School" is a school attended by boys only!

 

[EFLImpudence]

Yes, but that is not what was meant.

With plurals is not the same as for plurals.

 

[FrodoOne]

Yes, but that is not what was meant.

With plurals is not the same as for plurals.

Yes, I know - and I thought that I had pointed that out.
I just wanted to be explicit, in case it was read (scanned) wrongly by anyone.

 

[EFLImpudence]

Ok, sorry.

I can't find anything definitive on the net because I can't tell whether things are genuine or a mistake.

What about when boys is used as an adjective?

For example:
Possessive noun - That is my boys' school - i.e. the school of my two boys.
Adjective - That is my boys school - i.e. I own the school.

Eton boys school is not a school of only Eton boys, is it?

A cat basket or dog kennel has no possession indicator so why should the plural of the animal.