Learn Hardcore Swahili: Ninyi huchelewa darasani mara chache, lakini leo mmewahi sana. - You plural are rarely late to class, but today you have arrived very early.

In Ninyi huchelewa darasani…, it adds emphasis: You (people) are the ones who are rarely late.
Grammatically, it is not required. You could also say:
- Huchelewa darasani mara chache, lakini leo mmewahi sana.

In most normal conversations, speakers often drop ninyi unless they want to stress the subject (for contrast, emphasis, or clarity).

Why is it huchelewa and not mnachelewa?

Huchelewa here uses the hu- habitual tense marker, which expresses a general habit or repeated action.

Ninyi huchelewa darasani = You (plural) are late to class (as a habit / in general / usually).
Mnachelewa darasani would mean you are being late (now / these days) in the continuous present, not the neutral habitual.

Key points:

In the hu- habitual, the verb does not take a subject prefix:
- (Ninyi) huchelewa
- (Mimi) hulala mapema – I usually sleep early.
The subject can be stated separately (like ninyi) or understood from context.

So huchelewa here is chosen to express a general tendency, not a current ongoing situation.