*A window with no glass*
>! A lens is like a window but the lens of an eye has no glass.

*A student with no class*
>! Another word for student is pupil but the pupil of an eye has no class.

*A sea in any season*
>! "A sea" sounds like "I see", which the eye does year round.

*An epistle? No, a threesome*
>! Another word for epistle is letter. An eye is three letters.

The title: *An easy positive riddle*
>! "Easy" sounds like "I see". Eye sounds like “aye”, a positive answer, the sí of the sea.