When you go to Scoring Tools and "Score selected sets" you are not asking it to reassess the correctness of the answers students have submitted. You are asking it to look up whatever score has already been written to the database for each question, and then tally that up for the set.
In theory it could actually re-load every problem and reassess student answers saved in the answer log. The first concern with that is it could take a long time. Sometimes students explore and try wrong answers after they have full credit for a question. So you could not do what you want just by testing their most recent answer to each question. You'd have to test the entire history of answers they had submitted, and that could be a lot of PG processing.
But more importantly, there is in general no guarantee that the current form of the question is the same as the one back when a student entered an answer. Imagine a question that is problematic for some seeds. For one student, it is not problematic, and they get 100% early after the set opens. But then you get reports of other students having issues, and you alter the .pg file to fix the bug. Maybe that first student's answer is now incorrect with the new version of the problem code, and what you would like to do with re-scoring would unjustly take away credit from that first student.
There are several other reasons why conditions may be different with a particular problem between when a student answers it for 100% credit and when you would re-score. So even though WW does store all the answer history for each student for each question, there would be problems if this kind of re-scoring were implemented.
I'll add to Danny's suggestion about adding an 11th problem. You could keep the bad problem, but change its weight to 0. I think that would keep your set at 10 total points, but leave the one question in place to avoid the concerns Danny raises about deleting a problem from an active set. Whatever you end up doing, keep in mind any students who believe they have completed the set and have 100%. If you add an 11th problem, or somehow take away the "marked correct" status of that one problem, then such a student may be oblivious to the fact that they should go back to keep working on the set. And therefore you may need to do some outreach to the class.
WeBWorK Main Forum
Reversing "Mark correct"
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