Complex sentence: In TIMSS 2011, nationally representative samples of students in 63 countries and 14 benchmarking entities participated in the fourth grade assessment, the eighth grade assessment, or both, and in each country, almost 4,000 students from 150--200 schools participated at each grade.
Split into simpler sentences: In TIMSS 2011, nationally representative samples of students in 63 countries and 14 benchmarking entities participated in the fourth grade assessment, the eighth grade assessment, or both. And in each country, almost 4,000 students from 150--200 schools participated at each grade.