The Hurricane Detective Club and the Guardians of the Star Triangle (Etsiväkerho Hurrikaani ja tähtikolmion vartijat) by Jari Mäkipää.
The Hurricane Detective Club returns to school after a hectic summer vacation. The fall begins strangely when Principal McBride doesn’t show up for work. The club sets out to investigate the principal’s strange disappearance. The think it must have something to do with a strange symbol, the star triangle, a mystery the sleuths have tried to solve before.
As they investigate the star triangle, Jesse, Jenny, Caroline, and Matthias descend deeper into the dark secrets of Alder Ridge. Where has McBride disappeared to? What lurks behind the heavy steel doors in the cave? And is someone else trying to untangle the mystery of the star triangle?
The Hurricane Detective Club and the Guardians of the Start Triangle continues the exciting adventures of four young detectives. A hysterical supporting cast of old and new characters also comes along for the ride, including Anita Roquefort, and the strange master of comedy, Barry Buckshot.
From the Tammi/Elina Ahlbäck Agency Rights Guide:
This wonderful series began in 2004 with the publication of the Hurricane Detective Club Handbook. This fun guide gives useful tips to all budding detectives from setting up your own club to different disguises and secret codes. Jari Mäkipää has since written six detective novels, in which the four founding members of the original Hurricane Detective Club have adventures and amusing mishaps while solving mysterious incidents at their school.