What is grade a and what b in middle school?

There's no universal standard for what constitutes an "A" or "B" grade in middle school. Grading scales vary from school to school, district to district, and even sometimes from teacher to teacher within the same school.

Generally, however:

* A: Typically represents a superior understanding of the material, often in the 90-100% range (or sometimes even higher, depending on the grading system). Specific percentages can vary.

* B: Usually indicates a good understanding of the material, often in the 80-89% range. Again, this is a general guideline, and the exact percentage can differ.

To know the specific grading scale used in a particular middle school, you need to consult the school's grading policy, usually found in a student handbook or on the school's website.