The Third Form is the third year group at Malory Towers.
All About ItEdit
When Darrell is in the third form two new girls join: horse mad and tomboy Wilhemina "Bill" Robinson who has seven brothers and a horse called Thunder, and American fourth former Zerelda Brass who moves into the third. Another girl introduced in this book is aspiring opera singer Mavis, who was the new girl in the previous term, in the time between the books Second Form and Third Year.
Miss Peters is the Third Form's form mistress.