In every subject, at every level, we strive for mastery. Until a student has completely mastered one level, we don't move on to the next. Building later grades on a shaky foundation is a recipe for failure, and we consider it to be one of the greatest mistakes of conventional schooling.
What's more, we don't record failures, only successes. Another bad idea from conventional schooling is tarring a child with failures that follow him or her for life. If you see a subject on our transcript, you can be sure that the student has mastered it.
In other words, "A" is the only grade at Saint Columban's!