If you UPDATE existing IDs in the school table, you can define the pupil foreign key to have cascading updates, but if you ADD new values in the school table, and NO current record in the pupil table can possibly reference it (as of the time you create the new school record) how could you possibly have records in the pupil table automatically update?
Unless you want ALL records to update to the newest school id... (This would be done using a trigger, and is not something I think you would want to do...)
"Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson
"I have my standards. They may be low, but I have them!" - Bette Middler
"It's a book about a Spanish guy named Manual. You should read it." - Dilbert