"No person in the United States shall, on the ground of race, color, or national origin, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any program or activity receiving federal financial assistance."
Key words: receiving federal financial assistance. It's a private religious school, meaning it is barred from receiving government funding.
If you meant Title VII, that is generally prohibited, but not in the case of religious organizations. Case law has found that they are free to discriminate in their hiring practices (https://constitution.findlaw.com/amendment1/annotation08.htm...). It also doesn't stop a non-religious company from doing so but not disclosing their reason to the terminated employee, which they have no obligation to do in right-to-work jurisdictions.
Furthermore, reservations sit in a gray legal zone where they are considered semi-autonomous sovereign nations. In practice this exempts them from state law, but they are generally subject to federal law.
Observe this comment from the Equal Employment Opportunity Commission website (https://www.eeoc.gov/frequently-asked-questions-about-indian...):
"The EEOC does not have jurisdiction over charges of employment discrimination against federally recognized Tribes if the alleged discrimination is based on race, national origin, sex, color, or religion (under Title VII), disability (under the ADA), or genetic information (under GINA)."