3. Classrooms of bad students.
The Thai educational system stresses grouping students into classrooms of like ability, and then paradoxically gives them all the same lessons and tests. For the farang teacher, this can mean going from room of forty eager young learners to forty who can't yet read English, and certainly don't appreciate you trying to teach them.
The Thais place great emphasis on maintaining a jai yen literally a "cool heart", so losing your temper with an unruly class is virtually unheard of. Be aware that the Thai educational system places the responsibility for learning on the students, so that teachers are not held accountable for their pupils' failures. Instead, teachers will complain that the students are lazy or naughty, removing any possibility of poor teaching from the equation. This cuts both ways, as you are absolved of most blame for a poor lesson, but any successes can also be attributed to the strengths of the students.