9th grade is probably when I would start implementing the B or better policy, but I don't know exactly when the best time to start would be. Before then I would still expect them to perform, but would understand if they weren't mature yet, however, there would still have to be requirements to perform.
For each C, the student would be responsible for half the amount they would be responsible if they got a D or F. To compensate for the occasional C or even D, I would credit each student 5 percent of the D or F amount for each B and 10 percent for each A so five A's would cancel a C, ten A's would cancel a D and you can do the math for the B's. For F's, the student would not be allowed to use this credit without special permission.
While I wouldn't hold 7th and 8th graders responsible for their classes yet, I would allow them to start accumulating credit.
I would also have military style schools for those who don't perform by the end of 8th grade.