List to align, easiest way to realize

I'd start off with a data structure for the list you have. (This is assuming C# 6, by the way - for earlier versions of C# you wouldn't be able to use an auto-implemented readonly property, but that's about the only difference.)

public sealed class GradeBand
{
    public int MinScore { get; }
    public int MaxScore { get; } // Note: inclusive!
    public decimal Grade { get; }

    public GradeBand(int minScore, int maxScore, decimal grade)
    {
        // TODO: Validation
        MinScore = minScore;
        MaxScore = maxScore;
        Grade = grade;
    }
}

You can construct your list with:

var gradeBands = new List<GradeBand>
{
    new GradeBand(0, 5, 6.0m),
    ...
    new GradeBand(23, 28, 5.6m),
    ...
    new GradeBand(100, 100, 1.0m),
};

You'd probably want some sort of validation that the bands covered the whole range of grades.

Then there are two fairly obvious options for finding the grade. Firstly, a linear scan with no preprocessing:

public decimal FindGrade(IEnumerable<GradeBand> bands, int score)
{
    foreach (var band in bands)
    {
        if (band.MinScore <= score && score <= band.MaxScore)
        {
            return band.Grade;
        }
    }
    throw new ArgumentException("Score wasn't in any band");
}

Or you could preprocess the bands once:

var scoreToGrade = new decimal[101]; // Or whatever the max score is
foreach (var band in bands)
{
    for (int i = band.MinScore; i <= band.MaxScore; i++)
    {
        scoreToGrade[i] = band.Grade;
    }
}

Then for each score you can just use:

decimal grade = scoreToGrade[score];