班主任: a teacher in charge of a class
Relative explainations:
Examples:
1. 法语试卷是我们的班主任出的.
The French paper was set by our form teacher.
2. 如果你要了解儿子的一些情况你最好问一问他班主任.她最了解学生情况.
If you want to know anything about your son, you'd better ask the director of his class. She knows her students best.
3. 这位老师是5年级的班主任.
The teacher has charge of the fifth-year class.