Inverse Bag

Student.cs

public class Student
    {
        public virtual string FirstName { get; set; }
        public virtual string LastName { get; set; }
        public virtual IEnumerable<Classroom> Classrooms { get; set; }
        public virtual Teacher Teacher { get; set; }
    }

Teacher.cs

public class Teacher
    {
        public virtual string FirstName { get; set; }
        public virtual string LastName { get; set; }
        public virtual IEnumerable<Student> Students { get; set; }
    }

TeacherMap.cs

public class TeacherMap : ClassMapping<Teacher>
    {
        public TeacherMap()
        {
            Bag(x => x.Students, m =>
            {
                m.Lazy(CollectionLazy.Lazy);
                m.Inverse(true);
                m.Key(km => km.Column(typeof(Teacher).Name));
            }, map => map.OneToMany(a => a.Class(typeof(Student))));
        }
    }