When Will Hay tries to explain to the school governor the writing on the blackboard he mentions Joan of Arc in connection with the battle of Agincourt, but she would only have been 3 years old at the time.
To have rolled up a canvas as old as the Mona Lisa as is done in the film would have irretrievably ruined it.
During the school exams one of the St Michaels' pupils catapults an object towards the desk bell, but although one hears the ring of the bell the object ejected by the catapult is never seen striking the bell.