Sherlock Holmes and the Duke's son

Dr Huxtable has a school for boys in the north of England. When the Duke of Holdernesse decides to send his young son there, that is good news for the school. The Duke is a very important person, and Dr Huxtable is happy to have his son in the school.
But two weeks later Dr Huxtable is the unhappiest man in England. Why? And why does he take the train down to London and go to Baker Street? Why does he need the help of the famous detective Sherlock Holmes?
Because someone has kidnapped the Duke's son...