Power Query Formulas

AFFCF_Grants_Admin 

It's not necessary and better not to repeat Excel formulas literally. Group records by names without aggregation and within it work with each name as with separate table. Add Index as Test Number, with that for second row take difference in Score between it and Score in first row; in next column same for the third row. Please note in Power Query numbering starts from zero.

You may do latest transformations on separate query with only one name, after that copy/paste code under the grouping, here is only to change table name. Entire script looks like

let
    Source = Excel.CurrentWorkbook(){[Name="Tests"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}}),
    #"Sorted Rows" = Table.Sort(
        #"Changed Type",{{"Youth", Order.Ascending}, {"Date", Order.Ascending}}
    ),
    #"Grouped Rows" = Table.Group(
        #"Sorted Rows",
        {"Youth"},
        {{
            "Group",
            each [
                TestNumber = Table.AddIndexColumn(
        _,
        "Test Number", 1, 1, Int64.Type
    ),
    Diff2 = Table.AddColumn(
        TestNumber,
        "Diff2-1",
        each
            if [Test Number] = 2
            then TestNumber[Score]{1}-TestNumber[Score]{0}
            else null
    ),
    Diff3 = Table.AddColumn(
        Diff2,
        "Diff3-1",
        each
        if [Test Number] = 3
        then Diff2[Score]{2}-Diff2[Score]{0}
        else null
    ) ][Diff3]
        }}
    ),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Youth"}),
    #"Expanded Group" = Table.ExpandTableColumn(
        #"Removed Columns",
        "Group",
        {"Youth", "Date", "Score", "Test Number", "Diff2-1", "Diff3-1"},
        {"Youth", "Date", "Score", "Test Number", "Diff2-1", "Diff3-1"}
    )
in
    #"Expanded Group"