Do you need an apostrophe in pupils?

As always the use of an apostrophe is determined by the meaning you want to give the Word.

Pupils: more than one pupil e.g. There are 30 pupils in this class. (There is more than one)

Pupil's: owned by a pupil e.g. Have you seen this pupil's book? (The book that belongs to the pupil)

Pupils': owned by more than one pupil e.g. I want to talk to you about the pupils' behaviour. (the behaviour 'belonging to' more than one pupil)

NB Pupil can mean two things. A pupil can be a student at a school, which is how the Word is used in the above examples.'Pupil' is also the name of the transparent portion of the eye (the black dot in the middle). They are treated the same grammatically, but the example sentences would be different.