Katy Allen replaces Felicity Jones as Ethel Hallow from the second season onwards. According to Jones's mother, this was because she had missed home during the first season's production. Jones would later return as Ethel in the spin-off Weirdsister College.
The TV series never explained where Miss Bat went after the second season. However promotional material on CITV claimed she went to live in Inner Mongolia.
The similarities between this and the Harry Potter series have been noted for years. Kate Duchene was even offered a part in Harry Potter & The Chamber of Secrets, but turned it down out of loyalty to The Worst Witch franchise.
At the time of production, only four books in The Worst Witch series had been published. After the popularity of the TV series, Jill Murphy published three more books.
Drucilla only made one brief appearance in the fourth book. But after the popularity of the TV series, Jill Murphy featured her more in later books.
Jill Murphy approved of the TV series, preferring it to the previous television movie from the 80s. However she has said that she felt the earlier parts were better - before the TV series went past the books and had to invent new material. She was more involved in the 2017 reboot.
None of the teachers' first names were revealed in the books. This names Miss Cackle Amelia, Miss Hardbroom Constance, Miss Bat Davina and Miss Drill Imogen.
As this was a joint production between the UK and Canada, some episodes where the girls are notably outside Cackle's for a long time were shot in Canada.
Stephanie Lane who made one appearance in The Worst Witch later appeared in two more related projects: Weirdsister college and The New Worst Witch.
The fourth season was turned into the spin-off Weirdsister College, because producers felt it wasn't believable that Mildred would still be at school (Georgina Sherrington was eighteen at the time the third season ended).
Clare Coulter was the only non-British actor involved in the production (save for the material shot in Canada). When the series was dubbed over for the American and Canadian release, she dubbed herself using her natural accent.
During the Christmas pantomime, the characters are forced to put in a sequence of Ruby tap dancing to save time. Her actress Joanna Dyce is a trained tap dancer.
The TV series depicts Holly Rivers's character spelling her name 'Drucilla', and it appears as such in the books. However Jill Murphy later said it was meant to be spelled 'Drusilla', and it appears that way in the 2017 reboot.
Terence Hardiman plays the Chief Wizard in Season 1, but he's played by Richard Durden for his one appearance in Season 2. Hardiman returns to play him for Season 3.
The TV series actually fixed a continuity mistake present while adapting the books. In the first book, Ethel can still talk when she is turned into a pig. Yet when Ethel returns the favor and turns Mildred into a frog, she can't talk at all. In the TV adaptation, Ethel can't talk either when she is turned into a pig.