Answer: Federal government
Step-by-step explanation:
Based on the scenario in the question, the federal government would most likely determine whether or not the principal's action was constitutional.
This is a national issue and shouldn't be left in the hands of the local or state government. It should be noted that in 1962, school sponsored prayer in the public schools was banned by the United States Supreme Court, on the basis that the First Amendment was being violated.
The court later ruled that students can meet and then pray on their respective school grounds so far it's done privately and no one is forced.
Hence, the federal government would most likely determine whether or not the principal's action was constitutional.