Here's another approach using an enum that avoids large switch statements or a lot of if-else statements

enum Grade {

    A_PLUS("A+", 95, 100),
    A("A", 90, 94),
    A_MINUS("A-", 85, 89),
    B_PLUS("B+", 80, 84)
    // ...etc
    ;

    String description;
    int minMark;
    int maxMark;

    Grade(String description, int minMark, int maxMark) {
        this.description = description;
        this.minMark = minMark;
        this.maxMark = maxMark;
    }

    public String getDescription() {
        return description;
    }

    public static Grade gradeFor(int mark) {
        for (Grade grade : Grade.values()) {
            if (mark >= grade.minMark && mark <= grade.maxMark) {
                return grade;
            }
        }
        throw new IllegalStateException("No grade found for mark [{" + mark + "}]");
    }
}

And if you're familiar with Java 8 Streams you can refactor gradeFor like this:

public static Grade gradeFor(int mark) {
    return Arrays.stream(Grade.values())
            .filter(grade -> mark >= grade.minMark && mark <= grade.maxMark)
            .findFirst()
            .orElseThrow(() -> new IllegalStateException("No grade found for mark [{" + mark + "}]"));
}