Three short lines, and almost nothing happens. The speaker simply tells us that their last class has only five people in it. But the quiet way it's said invites us to read between the lines.
The word "last" can mean the final class of the day, or the final class before something ends, such as a course, a school year, or a stage of life. The poem doesn't say which, so both possibilities hang in the air. "Only" is the key word. It shows the speaker noticing the empty space and feeling a small letdown, maybe loneliness, maybe a sense that things are fading out. There's no complaint, though. The tone stays flat and understated, almost like a shrug.
The mood is quiet and a little wistful. With so few words on the page, the poem itself feels sparse, like the half-empty room it describes. What we're left with is a moment of noticing how thin things can get at the end.