There are basically two styles of lessons.
One style is the homeroom class (which teachers are there with the class most of the time). My class has 18 students in it.
Most teachers are around that number with two Chinese assistants.
Big group classes (usually about 30 minute lessons) are generally between 20 and 30 students.
We definitely train teachers to be able to handle the larger classes and with the M.E.F. system, once teachers get comfortable, the larger class style is generally easier (but higher energy) than small group classes.