"The force said “robust privacy and sharing agreements” had been approved by the Information Commissioner’s Office and development of the system done in collaboration with the Centre for Data Ethics and Innovation."
There is plenty of information about a child that, if made known to the parents, would wreak havoc on the child's life. For example, through metadata alone, it could be possible to identify that a child has LGBTQA interests/associations/etc. There are plenty of children where the parents knowing this could lead to them being punished harshly, or even on the streets. I don't think automatically sharing everything with parents is the expectation/panacea you seem to imply it would be.
Admittedly, I don't think the schools should have this information either.
...but not the parents.