“It seems that while the “quantity” group was busily churning out piles of work – and learning from their mistakes”Why would they care about quality? If they learned from their mistakes, they would make two or three pots, each one heavier than the rest (smart students would just make one item, making sure it weighed at least 50 kgs).
This probably would work if the teacher promised them 1 point for each mediocre item produced, 2 for each slightly less mediocre one, etc. with scores such that producing mediocre items at full speed at best gives them a low grade.
That gives the students both an incentive to start producing items soon (so that that can produce, as a safeguard, lots of mediocre items and get a decent grade) and to improve their technique (so that they can reach a higher grade, or finish earlier their work earlier)