Just Like Jennings by Anthony Buckeridge
Published by Collins, London (1961)
When Jennings and Darbishire happen upon a mysterious stranger, they are determined to prove he is a spy. Armed with a lemonade bottle and some chalk dust, their attempts to turn detective go astray, much to the amusement of the other schoolboys. Then, the appearance of an abominable snow-cat and that unfortunate incident with Mr Wilkin's antique vases. It's going to be difficult to stay out of trouble this term; but isn't that just like Jennings? - Fantabulous!