YA: unknown prince hiding in nantucket, big cannon

[Michael]

YA book read in late 70s
may be part of a series
young boy who doesn't know he's a prince hiding in a school in Nantucket.
young female friend who helps out
there something about a big cannon either firing from Nantucket or onto the island.

[Mai]

Searching on WorldCat for juvenile novels published before 1980 with the key words "Nantucket" and "school" The Wolves Chronicles Series by Joan Aiken came up. There's a book called Nightbirds on Nantucket that mentions a giant cannon.
You can read some more about the book here: https://en.wikipedia.org/wiki/Nightbi...

The main character is a girl but just in case!