Undo the last mark
Every mark you make raises a strip under the grid header naming what changed — the learner, the core value, and what the cell was before and is now. Press Undo that and the cell goes back to exactly what it replaced, including a blank.
It holds one change
The strip is one change deep, and it is the last single cell you touched. It clears when you switch grading period, when you set a whole column with Everyone, and when you leave the page.
That means a column set with Everyone cannot be taken back in one press. If you set the wrong mark on a column, set the column again to the right one, then fix the individual learners.
Recent changes
Below the grid, Recent changes lists the last eight marks on this class, across grading periods. Each row names the learner, the core value with its period, what the mark was and what it is now, how long ago, and whether it was you. A cleared cell shows as a change to blank. A learner who has been taken off the roster shows as "A learner who has left".
The trail underneath
Every mark and every clear is recorded whether or not you use undo, along with what it replaced and who made it. Recent changes is the readable top of that record; the record itself is kept after those eight rows have scrolled past.
If a cell looks wrong and you did not change it, check Recent changes first: it will name the mark it replaced.