Classes go from M.1-M.1 [M="Matthayom"], which are more or less the same years that other countries have junior high school and high school. The groups are pretty stable from M.1 to M.3, at which point they get another "entrance" opportunity to upgrade their high school; therefore, most schools will face a "brain drain" of their best and brightest between M.3 and M.4 (though they will get an influx of the better students from the lower schools).