In part at least, this goes back farther than 1978. Read about Bob Jones University vs.the United States, circa 1970. In that, the ruling from the IRS came down that, since Bob Jones University practiced discriminatory policies, they were not entitled to the tax exempt status.
Note: Bob Jones University, while admitting non-white students, refused admissions to those in mixed race relartionships.
The above stated laws most likely also apply to lower level education (grades 1 through 12).
I'm not overly familiar with the legal workings of the U.S.A., and am only offering surface information, hopefully someone else can provide more information and detail(s).