With both "mandatory" and "obligatory" only the present subjunctive (uninflected base form) is used regardless of the tense of the main verb.
The use of the uninflected base form means there is lack of the usual concord between subject and finite verb in the 3rd person singular present. Also, there is no distinction between present and past tenses.
- It's mandatory/obligatory that she take that exam.
And in the negative, for example:
- It's mandatory/obligatory that she not take that exam.
The reason the verb does not take an ‘s’ is because this construction is what is known as the mandative subjunctive (also known as the mandatory subjunctive).
The mandative subjunctive is used with clauses that usually, but not always, begin with that and express a demand, requirement, request, recommendation or suggestion. It is indicated by the use of the base form of the verb (love, skip, jump) with singular subjects.