Just say “when the exams are over” or “when the exams have ended.”
Both are idiomatic in a sense, because “exams” is standing in for “exam period.” In the first, “are over” is a predicative completion describing the state of the period. In the second, “have ended” is a compound verb referring to an action - the running of time during the period. That’s why the auxiliary verb is “to have” rather than “to be.”
You can also omit the definite article: “ ... when exams are over/have ended.”
If you need to distinguish between all and some, say “... when all the exams are over/have ended,” with stress on “all” in spoken language.