May a Seminarian Say Mass?

If by seminarian you mean someone studying to be a priest who is not yet ordained, then, no, the Mass would not be valid. Only a validly ordained priest can consecrate the Eucharist. If this is the case, then the bishop should be notified. Simulation of a sacrament is a grave violation of the Code of Canon Law (can. 1378 §2,1).

If by seminarian you simply mean someone who is not yet completed his degree but has already been ordained a priest, then, yes, it is valid.