I think the children nowadays are very 早熟 (perhaps because children have to face competition in very young ages, e.g. for competition for places of good schools). Therefore, I believe those children in secondary school (even if in Form 1) (and even some senior primary school students) could make their own rational choice (of course, they may be lack of experience when compared to those much older (e.g. 40 years old)). As such, I see this case as 2 rational people doing something like a pair of couple. Therefore, I don't know if the girl takes the initiate (it is heard that the chance of successful "chasing" would be higher if the girl takes the initiate). I personally don't mind others fall in love with each other even if there is a great difference in ages. However, If the teacher really did something against the law (衰11), he should be prepared to bear the legal consequences.
P.S.: I really feel that it is strange for the teacher to give $100 to the girl after 開房. If the teacher just wants to give some $$ to the girl, why just pay the $$ after 開房? Is it because the girl knows that she provides a good to the teacher, and thus ask for compensation? Or is it because of the teacher just treat the girl as a sexual service provider???
The question is silimiar to the relationship between the doctor and the patient. (Or priest and altar boy ) The relationship is one of trust.
The girl is NOT old enough to make a rational choice! By placing one girl underage to make the choice, we will have to place all the girl of the same age and older to make the same choice because we have no mind reader to check for maturity. If the age is in disputed, people can always appeal to the representative in the government for a change of law.
When her guardian places her under the trust of the teacher, They probably expect the teacher to behave professionally and do not take advantage with his position as a teacher.
A similiar situation arise when a patient listens to a doctor.about his sickness. The patient expects the doctor to behave professionally and not tell lies for whatever reason the doctor has.
The deeper impact behind all these is that if doctors tell lie, would anyone believe the doctors?
If the teachers are allow to "temper" with his/her student, who would leave their children to the teacher? (Would you leave your childen beside a heavy machinery in the factory?)
Therer is indeed a practical reason behind the law.
But, I think it is possible for the girl to 引誘 the teacher. It is particularly strange that the teacher give the girl $$ after having sex (or just 開房without doing anything?? God knows...)....