Is it a crime for a 27 year old professional teacher to date a 19 year-old j.h.s girl who is supposed to be in S.h.s but very poor in academic with intention of marrying her?

Is that what he told you. He will marry you ? I can think of only one reason a 27 yr old man would be interested in a not very bright 19 yr old.

It's a breech of ethics. Because he is in a position of power, he could lose his job.

In my area, most people in JHS are 10 through 13 years old.

It is unclear if this is a 27 year old, or the 19 year old, but I see no crime here on the face of it. The devil is in the details of the facts of the full story, which we don't have.

The crime is that the 27 year old is in a position of authority.

It is unclear if the student and teacher are in even in the same school.

The issue here is their age, and maybe the mental capacity of the 19 year old.

In another context, if this was university, and a teacher was dating a student. (happens some in US, happens every day here China, there would be no law issue, and if not his student, not even a ethic issue in most schools.

If he is HER teacher, then it will could be a crime, but it would depend on locatoin and how the law is written.

Is it a crime for a 27 year old professional teacher to date a 19 year-old j.h.s girl who is supposed to be in S.h.s but very poor in academic with intention of marrying her?

First, any question on law needs to include your general locale as laws vary by area. Second. A 19 yr old still in grade 9 or earlier (JHS usually go up to 8 or 9) is more than academically challenged. I would suspect there is a mental incapacity there. And that may bear on the legality. Third, there is the question of whether the 27 yr old is her teacher. If he is, it is a serious breach of ethics and could cost him his career if not a criminal charge.

On the flip side of this, a 19 yr old is, in most cases an adult and free to date whoever she chooses to except for the conditions cited earlier.

So there are several questions that need to be answered here before we can have an idea of whether criminal charges might be prosecuted.

It is unclear if this is a 27 year old, or the 19 year old, but ...

If OP is the teacher, I hope he isn't an English teacher.