No blacks, no asians, no hispanics in high school?

No, that's not it. The movie took place in the late 50s and back then, high schools were still very much racially segregated. You need to keep in mind that the Supreme Court had ruled on Brown vs. the Board of Education only several years earlier (in 1954), but many localities, especially in the South, were still very resistant to integrating their schools--from elementary school on up to college. If there were any minorities in these schools, they truly were in the minority. I'll give you an example: my mom attended high school in West Chicago, Illinois, from 1957-1960, and she was one of only about 2-3 Hispanics out of an otherwise white student population. Her classmates did accept and befriend her, but she was truly in the minority. Of course, it's a much different story today, but this movie is a portrait of life in another era and I think the producers wanted to faithfully portray things as they were back then.

There is a black couple at the dance.

In addition to the segregation someone mentioned, consider this movie was made in the 70's about life in the 50's.
The cultural makeup of the nation was VASTLY different in both if those eras from where it is now.
There's a reason the term "minority" came into being. LoL

nope, none at all.

Swing away, Merrill....Merrill, swing away...

I went to junior-high school in Massachusetts in the 70's and we didn't have one black student. Some neighborhoods just didn't have blacks. Eventually there were only 2 black students.

Yeah, could you possibly imagine filming this movie today? I bet you couldn't even get the kids to put their cellphones down!

I don't have that much of a problem with there not being many minorities in movies like Grease. 1978 was a long time ago and I would imagine that schools during the 50's in most of the U.S. were pretty white, and as another poster said earlier there was still racial segregation at all levels. My problem is that in 2015 there are still no Mexicans in most movies or television. It's like we don't exist. I personally can't stand that they force Asians or Blacks into roles were there would not be any, such as in medieval movies. Give me a break. I'm 50 years old and I don't see things changing anytime soon. When most people talk about discrimination in Hollywood they're always screaming that there aren't any women directors or producers or that a western movie had no blacks. Think about this: in most U.S. made Westerns there were often no Mexicans at all. How is that even possible? Of course when they do appear they're drunks and bandits only, and can't shoot straight. It took Spaghetti Westerns to give a more realistic view of the South West. Embarrassing.

It would've been more politically correct to add minorities but not very realistic. I don't think minorities were very prevalent in the 50's back then and if they were present, I can't imagine them being accepted and may probably have remained in the fringes. Sad but true.