Exams are one of the settings where there is natural social distancing so I'd have thought lower risk than normal school, to be honest, especially if everyone wears a mask. They're not supposed to be talking either, which would reduce risk.
If you had good ventilation, exams could be pretty low risk. They did them last year in Germany IIRC without a spike in cases.
Whether they should take exams at all given disrupted education is another question.