U.S. high schools aren't showing much improvement when it comes to preparing college-ready graduates.

That's according to the Iowa-based nonprofit group that administers the ACT college entrance exam.

The group says only about 40 percent of graduating high school students who took the ACT exam this year show a "strong readiness" for college in most subject areas. Meanwhile, 31 percent of these students aren't meeting readiness levels in any core subject areas.

These levels are about the same as four years ago. The data also shows negligible changes among ethnic groups since 2011, with white and Asian American students still dramatically outperforming other ethnicities.

ACT Chief Executive Officer Jon Whitmore called the findings a "wake-up call" and said the education system must do more to prepare students.