It’s the one German boarding school novel and I loved it to pieces as a kid. As all Kästner books except for Lottchen it features a fairly transparent self-insert, but it’s fine, imo. Most boarding school novels do this, to be honest.
What I appreciate most, I think, is how real it feels. The kids have worries and problems, and they are vital and real and are never minimised. No matter if one of the boys feels scared and neglected or if one has not enough money to go home for Christmas: they have existential feelings and the book does not shy away from them. There are good teachers and stern teachers, older bullies, younger bullies, protectors in unlikely places, artsy students, and above all: friendship.
There are a couple of passages in this book that touched my life, repeatedly, when they showed adults behaving with responsibility and kindness when I felt that I had nobody who I could count on to do the same for me. I first read this book when I was nine, and then read it many more times until I was ~13. I’m hardly objective here, but it feels like home and I love it.