You can do extra credit with a B or an A.
It seems the fear is canceled if your grade reaches C.
It's "Fear of Failing Classes". Looks like I failed to read thoroughly. You have to ask the teacher for extra credit work after finishing the day's homework.
I plan on finding out this evening.
Edit: I had to wait until the end of the school day for the fear to trigger, and then do the homework at school, but then I was able to ask Rita Coombes for extra credit.
The next day the fear didn't trigger at the end of the day so I sent my teen Sim home. It triggered while she was doing her homework, but while Rita became a stayover house guest while my teen Sim was doing homework., I was not able to ask her for extra credit after finishing homework.
I think it may be bugged, despite that, unless you can't get the fear without High School Years, and I suspect the Fear is tied to the Troublemaker Scenario (where the teen sim starts with an F), which is available to players without High School Years. Also, if you can get the fear as a child (by slacking off at school or making friends at school enough times in a row to get a D) it's bugged, as there is no way to ask a teacher for extra credit work.
