Why is my if....elif statement always returning the second arguement?

I am writing a code that calculates a curved grade average based off of the score, mean of all scores, and the standard deviation. The code should output "A", "B", "C", "D", or "F" but I am only getting "B" for all scores.

Here is the piece of code that is the problem:

def curveGrades(mean, sd, gradeList):
    m = mean
    for item in gradeList:
        if item >= m + 1.5 * sd:
            grade = "A"
        elif m + 0.5 * sd <= item < m + 1.5 * sd:
            grade = "B"
        elif m - 0.5 * sd <= item < m + 0.5 * sd:
            grade = "C"
        elif m - 1.5 * sd <= item < m - 0.5 * sd:
            grade = "D"
        elif item < m - 1.5 * sd:
            grade = "F"
        return grade   

It should print all the different letters based of the values in gradeList, but it continues to give me "B" for every score.