The ‘@id’ and ‘@type’ cannot be a NCName, these are IRIs.
you should do:
prefix study: <https://study.org/>
SELECT ?id ?type ?subject ?result
{ [] study:exams ?id .
?id study:subject ?subject;
study:result ?result ; rdf:type ?type .
}
you will get IRIs for subject and for type of entities where IRIs depends of JSON-LD context.
HTH