Every three years,the OECD tests the skills and knowledge of 15-year-olds around the world.In 2015,more than half a million teenagers, representing (represent)28 million students in 72 countries,took the test.

空格处的现在分词在句中充当什么成分？感觉不太像是作teenagers的后置定语，并且为什么不能用过去分词或是动词过去式？