Myrtille, no the school does not have to take it seriously, in fact quite the reverse. As I said, parents can pay to have ANYTHING written in private report. They can just get one after another after another until they get one the like.
So, I am sure you appreciate, it would be impossible for schools to act on them, some of them give information directly opposedto the schools own assessment!
The LEAs I have worked for have always had a policy of totally disregarding them. The are often filed on the premises, and staff can read the if they have time, but actually they are used more to inform us about the parents, than the children.
Only the assessments comissioned at he school by approved assessors are taken into account.
I'm sure you can see it would be completely unmanageable to read and act on all the private assesments that come pouring in to a school. Even to find time to read them would be impossible for teachers, and even if you did, you would not remember the hundreds of pages produced, and even if you did the value of that information would be very highly suspect.
( incidently, the one student whos assessments I did browse through once contained the diagnosis " Sharon has had too many assessments! - this was true, she had a whole filing cabinet draw to herself, her parents were known to have a serious problem)