Calculating Difference from Previous Row

You can do this all in Power Query

• Group by Unique ID
• For each subgroup
  • Add an shifted (offset) Grade value column
    • (more efficient subtraction than using an Index column)
  • Subtract the Shifted Grade Value from the Gr ade Value
• Re-expand the table

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyNlHSUQJhx9KS0tw8BUOlWB24uDlC3AhZ3BSIgwuKMvPSoepNzcwtgGKGRugGwSRM0E1ClkAxytLQwNAQ1W6cEkYoEhYoRsUCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Unique ID" = _t, #"Grade Value" = _t, #"Half Term" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Unique ID", Int64.Type}, {"Grade Value", Int64.Type}, {"Half Term", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Unique ID"}, {
        {"Half Termly Changes", (t)=>
            let 
                //add shifted grade value column
                //Then subtract from grade value
                #"Shifted Grade Value" = Table.FromColumns(
                                            Table.ToColumns(t)
                                            & {{null} & List.RemoveLastN(t[Grade Value])},
                                        type table[Unique ID=nullable number, Grade Value=nullable number, Half Term=nullable text, Shifted Grade Value=nullable number]),
                #"Change" = Table.AddColumn(#"Shifted Grade Value","Half Termly Change", each [Grade Value] - [Shifted Grade Value], type nullable number),
                #"Remove Shifted" = Table.RemoveColumns(#"Change",{"Shifted Grade Value"})
            in 
                #"Remove Shifted",
                type table [Unique ID=nullable number, Grade Value=nullable number, Half Term=nullable text, Half Termly Changes=nullable number]}}),
    #"Expanded Half Termly Changes" = Table.ExpandTableColumn(#"Grouped Rows", "Half Termly Changes", {"Grade Value", "Half Term", "Half Termly Change"})
in
    #"Expanded Half Termly Changes"

Results