Same in China (Shanghai). Same 30 to 35 students in the same class would do 1st to 6th grade. Then you'd usually go to a different school for 7th to 9th grades, then take a test to get into a high school for 10th to 12th grades. All three would have the same class you'd stick with, usually with way more intra-class interaction than inter-class interaction.

Sports was usually one thing that was more inter-class, but that was it.