Grades are almost completely relative, in effect ranking students relative to others in their class. Thus extra achievement by one student not only raises his position, but in effect lowers the position of others. ↗
As an example, one of the schools I have been studying is too small to compete effectively in most sports, but participates with vigor each year in the state music contests. ↗
Cultural dominance of middle-class norms prevail in middle-class schools with a teacher teaching toward those standards and with students striving to maintain those standards. ↗