SC - Subject Verb agreement when the subject is collective n

This can be a better eg.

The staff meets tonight. - Staff collectively

The staff have different opinions. - staff represents individual staff members

What are your comments on this?

Here is an example, which I studied in a grammar book.
as singular
The Committee has issued its report.

now as plural
The Committee are divided on one minor point.

But to be consistent we must say
The Committee has appended a note to its report. ( not their ).

Actually, a collective noun takes a singular verb when the collection is thought of as one whole; plural verb when the individuals of which it is composed are thought of.

here in your question's case, I think "The group of children" is thought of as one whole group who hasn't seen the ocean. It doesn't matter whether the group is from school or not, so the correct answer is

"The group of children from the school has never seen the ocean."

what matters most is that it is thought of one whole. ( all the group hasn't seen the ocean )

Hope it helps, do correct me if I am wrong ( would really appreciate ) with explanation.

And yea as far as

"The staff have different opinions"
is concerned it is plural because individuals may have different opinions ( all of the staff as a whole can't have different opinions ). Because as I wrote before "plural verb when the individuals of which it is composed are thought of"
, and here individuals of The staff are thought of.

Hope it helps