(If you don't like watching videos, here's Diane Ravitch's transcript.)
The classroom appears to have a student body made up of about 14 blonde and 8 brunet white kids. (There seems to be one black student on the edge of one shot, but he doesn't seem to show up in other shots -- this video could represent multiple classes.)
The racial makeup of the city was 91.0% White, 2.2% African American, 0.1% Native American, 4.6% Asian, 0.6% from other races, and 1.5% from two or more races. Hispanic or Latino of any race were 2.0% of the population.
If it's Iowa and the second biggest ethnic group is Asians, it's pretty high end. Also, the students in the video are slender, so this is clearly an upscale school.
From Zillow, here are the demographics of Johnston High School:
Only 13% of Johnston High students qualify for reduced price lunches compared to almost half of all students nationally.
Wikipedia doesn't give the 2010 income figures for Johnston, but back in 2000: "the median income for a family was $97,322." Nearly six figures in 2000 in Iowa must put that community at the 95th percentile or higher. A 2011 US News article listed the Des Moines metropolitan area, with its cost of living at only 90.6% of the national average and median income of $56,576, as having the highest cost-adjusted standard of living in the country. And Johnston is gold-plated by Des Moines standards.
Bill ends his video about Johnston High at 7:58 with the assertion:
every classroom could look like that
Of course, nobody, Gates or his TED audience, gets the joke.