"Wameiba vitabu vya wanafunzi"

Translation: They have stolen the books of the pupils

[CliffordPereira]

"They have stolen the books of the pupils" or simply "They have stolen the students' books"

[garpike]

"Books of pupils' " would just about work (although hardly idiomatic), but 'books of pupils' reads like books containing lists of pupils (or, perhaps, pictures of eyes).