When viewing students progress, the circle indicates the furthest puzzle that a student has attempted in that course. The number inside the circle refers to the puzzle number of the stage where the furthest puzzle was attempted.
White space indicates a student has not completed the puzzle. Light green indicates that a student has completed the puzzle, using more blocks than what Code.org suggests for the optimal solution. And bright green indicates a student has completed that puzzle with what we consider the optimal number of blocks.
If your student doesn't have a circle on their progress, that means they have attempted the final puzzle of that course. And once a student has attempted the last puzzle, that will always be the furthest puzzle that they’ve completed so the circle will not reappear.