"The students sleep."
discipulos is an accusative form. It is used in most cases as a direct object, the 'receiver' of the verb.
discipulos habent -> 'They have students.' (The students are being 'had').
discipuli here is nominative and is the subject of the sentence, the ones doing the verb.
discipuli habent -> 'The students have (something, I don't know, just an example, but the students are doing the having)'
The difference is similar to in English with pronouns. Example: they/them. 'They have students.' vs 'Students have them.'