I have read the first three volumes of this. My friend has them.
It's not that the girl is sleeping/or has slept with the teacher. She used to be tutored by him, and she fell in love with him. When she told him how she felt, he said that he just loved her like a little sister. He actually thinks he might love her back, but is afraid to admit it becuase of (1)the ages, (2) she's a student and he's a teacher, (3) has pressures of getting married from his 'father'.
Overall it was pretty funny and sweet book. It's definitely not my favorite, but still ok. Thats my opinion! ^-^
Yeito