[sammyg2]

Not sure how many students take the test, but here's something about it:
From Wikipedia, the free encyclopedia

The Iowa Test of Basic Skills (ITBS) are a set of standardized tests given annually to school students in the United States. These tests are given to students beginning in kindergarten and progressing until Grade 8 to assess educational development. In Grade 9, the students are examined using the ITED program.

The ITBS was designed by the University of Iowa's College of Education, as part of a program to develop a series of nationally accepted standardised achievement tests.

The ITBS covers numerous areas of educational knowledge, including English language use, math concepts and problem solving, social studies, science, geography and map use, and source use.

The ITBS is used in the majority of schools in the State of Iowa, both in the public and private education sectors. The tests have been widely used in other regions of the United States.

A quick google search didn't tell me exactly how many tak it, but it is used in illinois, Washington, South Carolina, Missouri, Montana, Nevada, at least parts of Texas, Georgia, etc.
It appears it is a nationwide test, and has pecome much more popular due to the "no child left behind" act. Here's a snip of what I found:

Annual test now has impact beyond just student scores
BY MIKE BUFFINGTON AND ANGELA GARY
If the power of the pen is like a sword, then the power of a No. 2 pencil held by a third-grader is a Sherman tank. At least that's a prevailing view of many school administrators as students begin taking the annual Iowa Test of Basic Skills (ITBS).
This week, students in local elementary and middle schools began taking the test, which has in recent years become THE standard by which many have come to judge schools and school systems. As a nationwide test, the ITBS forms a comparison of students to their peers from all states, a result that is more accurate than many other tests, say some school leaders. And because parents now place such importance on standardized test results, education leaders have begun to emphasize the ITBS within their schools.