Treasure Hunt Mystery in a school for a time capsule type thing hid by the school's founder. Spoiler ahead.

[Declan]

I don't remeber a lot about the book but it was aimed towards kids aged 10-12. I read it somewhere between 2012 and 2016
The protagonists were a girl and a boy, who were either siblings or friends.
The treasure hunt was based around different parts of the school, like a bust of someone and a bell in a tower. There were illustrations accompanyig these clues.
The cover of the book shows the two kids climbing up a tower towards a large bell with writing or inscriptions.

SPOILER: The treasure is somehow related to Picasso, who the founder or Principal knew. Theres a second book where the plot remains mostly the same but the treasure belonged to a famous magician.

Thanks if anyone knows what I'm describing.

[Becca]

The Lost Treasure of Tuckernuck by Emily Fairlie

There isn't a Google Books preview available but there is a copy on archive.org - it looks like the treasure is a Picasso painting: https://archive.org/details/losttreas...
The sequel is The Magician's Bird