The problem many have with such a relationship is the position of the teacher and that of the student.
A student is by definition, in a subordinate position to the teacher, who often has power over that student.
With an unequal relationship already established, the question then is: is this relationship actually consensual, or is it in one that the person in power (teacher) is forcing upon the other.
Unfortunately, there are a lot of landmines waiting to explode on the author when trying to establish a teacher-student relationship, and even good writers fail to successfully execute these at times.
If you do decide to do this, I suggest getting a lot of different readers to test read this romance, because you most likely will find that readers see aspects of the relationship problematic that either you failed to see yourself, or that you did not think would matter.
Edit: Please also review this thread:
This has a lot of people’s opinions already recorded on the age gap issue.