
The Worst Witch Strikes Again by Jill Murphy, 1980.

It’s a new term at Miss Cackle’s Academy, and Mildred Hubble is back! In spite of the headmistress’s gratitude toward her for saving the school the previous semester, it isn’t long before Mildred finds herself in hot water again. She still manages to get her lessons wrong, and admittedly, sometimes goofs around or does naughty things out of boredom.
In an effort to teach Mildred some responsibility, Miss Cackle makes Mildred responsible for the new girl at school, Enid Nightshade. Mildred is supposed to show Enid around and make sure that she knows what she’s doing. Miss Hardbroom, the potions teacher, is dubious about this because it often seems like Mildred herself doesn’t know what she’s doing. Although Miss Cackle is hopeful that this new responsibility will give Mildred the incentive she needs to improve, so she can set a good example for Enid. However, what no one seems to consider is whether Enid is going to be a good influence on Mildred.

During their chanting lesson, Enid sings funny on purpose, and it makes Mildred laugh, getting her into trouble again. Enid also keeps a pet monkey in her room, even though only cats are allowed at the school, and Mildred accidentally lets it loose! After the monkey causes chaos in the school, it is revealed that the monkey is actually Enid’s cat. Enid changed it into a monkey for fun, but Mildred is blamed for doing it. Mildred can’t bring herself to tattle on Enid. Worse still, Mildred’s best friend, Maud, takes offense to Mildred spending so much time with Enid instead of her. Mildred can’t help it because she knows that Miss Cackle is expecting her to help Enid, but Enid keeps getting her into trouble, and Maud starts hanging out with the insufferable Ethel instead.
Enid continually gets Mildred in trouble, putting her in danger of getting expelled, while nobody seems to suspect Enid. Yet, Mildred has trouble getting angry with Enid because she doesn’t seem to mean the trouble she causes. At least, she doesn’t seem to mean for Mildred to take the blame, although she doesn’t step up to take responsibility and clear Mildred’s name, either. It seems like, rather than setting a good example for Enid, Mildred’s lessons in responsibility will be learning how to stand up to Enid and her escapades!
The book is available to borrow and read for free online through Internet Archive (multiple copies).
My Reaction and Some Spoilers

The idea of a teacher helping a student to learn responsibility by making that student responsible for a younger student is a classic boarding school story trope, dating back to Tom Brown’s School Days. However, this story humorously inverts the concept because Enid is a mischievous girl who ends up getting Mildred into more trouble than Mildred ordinarily gets into by herself! I was a bit annoyed at Enid because of how she causes trouble for Mildred and doesn’t take responsibility for it at first. The story makes it clear, though, that Enid doesn’t realized just how much trouble she’s causing for Mildred at first. When Enid realizes that Mildred is in serious trouble with the headmistress, she’s prepared to take responsibility.
It takes a nasty trick played by Ethel to really bring Mildred, Enid, and Maud together. Mildred and Enid pull off a wild stunt to get out of Ethel’s trick, and Maud stands up for Mildred and Enid, helping them to pass off the escapade as an ambitious tribute to the school. Fortunately, it’s enough to smooth things over with both the headmistress and the strict potions teacher!
From this point on in the series, Mildred, Maud, and Enid are all friends with each other.