US Readers might have a tough time with this.
First, that three stars is for US audiences only. I understand what Willans was doing -- I really do. He's emulating the style of a schoolboy, and one who's flunking his classes, at that. Unfortunately, my school pounded "correct English & perfect spelling" into my brain HARD, so reading Willans's Molesworth's writing (typical kid with very...creative...spelling) was near-impossible for me. I got exhausted going back over every sentence several times to figure out what was being said. All the British schoolkid slang didn't help, either, and moreso because it's probably outdated slang. Worse, the book is based solidly in the British school system of the 1950s, so most of it whooshed waaaaaay over my head.
Before ANYONE gets mad at this ignorant US person slanging on their beloved book -- the fault is entirely mine. I'm a Yank who's too firmly entrenched in US culture. There were some points where I understood what The Terror of St Custards was saying, and those points were hilarious. The points where Molesworth slips back into correct grammatical English were hilarious -- his parody of a Latin Classical play had me giggling. Yeah, we got that stuff forced into our heads in high school (a Catholic private school). I loved realizing where JK Rowling got the name of Hogwarts! There were definitely parts where the British school experience intersected with US school experience, and those were likewise funny as hell -- again, when I finally figured out what Molesworth was saying.
US folks, it's similar to a prose Calvin & Hobbes without the toy tiger and with horrible spelling. If for no other reason, check this book out for Searles' wonderful illustrations. I really, really wanted to enjoy the whole book. Sadly, my American background blocked it, and the deliberately-bad spelling & grammar only made my eyes hurt.
I'd stumbled on the TVTropes entry for this series, found the listed tropes and descriptions of various scenes funny, and checked out this book & its predecessor (Down with Skool) on the strength of that. However, the culture gap was too great for me, the Molesworth writing style too much.
British folks, you'll likely love this book. US folks? Probably not. I'm too much a fule to kno.