How to evaluate test results from multiple students?

Aah, gotcha. Sorry.

Try this instead:

// Call this 'stephAnswers'
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WKjRU0lEyVIrVATKNgMwkCNMYyCwpKk2F8ExAEjmJydkQrimQa2mpFBsLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [question = _t, answer = _t])
in
    Source

// Call this 'stephTest'
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WKjZU0lEqBBGGSrE6ML4RkEhC4hsDiZKi0lQkIROQkpzE5GwkMVMgYWkJETBCM9cIzVwjmLlpiTnFqUhiIIPLMzJLkMVABhsaQE0yRjPZGM1kY1QXxwIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [student = _t, question = _t, answer = _t]),
    addStudentQ = Table.AddColumn(Source, "studentQ", each Text.Combine({[student], [question]}, "-"), type text)
in
    addStudentQ

// Call this 'stephRootTable'
let
    Source = Table.Distinct(Table.SelectColumns(stephTest, "student")),
    addPossQuestion = Table.AddColumn(Source, "possQuestion", each stephAnswers[question]),
    expandPossQuestion = Table.ExpandListColumn(addPossQuestion, "possQuestion"),
    addStudentQ = Table.AddColumn(expandPossQuestion, "studentQ", each Text.Combine({[student], [possQuestion]}, "-"), type text),
    chgTypes = Table.TransformColumnTypes(addStudentQ,{{"possQuestion", type text}})
in
    chgTypes

Apply all three to your model and relate as follows:

stephAnswers[question] > stephRootTable[possQuestion]

stephTest[studentQ] > stephRootTable[studentQ]

Add a new custom column in stephRootTable as follows:

..score = IF(RELATED(stephTest[answer]) = RELATED(stephAnswers[answer]), 1, 0)

Chuck it all in a table as follows:

This should be super-scalable as you're combining lower-resource functions of Power Query with the power of relationships to avoid costly PQ joins.

Pete