For the full article explaining why a school would use this test, check out this document.  why_use_the_isa

Here are some key points from the article: 

• The ISA adds value to internal school assessments.
• The ISA is a high-quality assessment.
• The ISA is an international test.
• The ISA Individual Reports tell parents what they want to know.
• The ISA Class Reports provide detailed diagnostic information to teachers.
• The ISA School Reports allows schools to benchmark themselves against other schools.
• A student’s ISA scale scores can be transferred to a new school.
• The ISA Series E test was administered to over 28,000 students in 173 international schools in
  2006−7, providing a sound basis for comparison of results at each grade level.

Visit the ISA website for more information. 

Source: "Why use the ISA?" published by ACER for ISA.

Here are some informative PDF files of sample ISA questions to give you an idea of what the questions and format of the ISA will be like.

Sample Mathematics Items - Grade 3  Sample Mathematics Items - Grade 5  Sample Mathematics Items - Grade 7  Sample Mathematics Items - Grade 9/10	Sample Reading Items - Grade 3  Sample Reading Items - Grade 5  Sample Reading Items - Grade 7  Sample Reading Items - Grade 10
Sample Writing Items - Grade 5

This test, which has been created by the Australian Council for Educational Research (ACER), is designed specifically for use within an international school.  The test is administered over two mornings and will evaluate students in the areas of mathematic literacy, reading and writing.  Since the test questions are not based upon the curriculum of any single country or school system and instead a compilation of concepts and skills taught to students around the world, this assessment will allow us to compare the results of BFIS students with same-age peers in other international schools.  Last year, there were 173 international schools administering this test to their students, which will provide a far more meaningful point of comparison. BFIS students in grades 3-9 will take the ISA.