There is a huge potential for disaster and drama for the both of them, even if the relationship works perfectly, due to the situation. How long are you going to be in that school? If it's something like a year, I would suggest being close friends for now, and pursuing a relationship once you're not a student there anymore. If it's more like five years... I would give them an option to weigh in on that decision, but expect them to be very cautious.
It's not just the student-teacher thing that could go wrong. In other people's eyes, it would also be adultery. People might ask to change teachers. People might ask to change schools. He could get fired. It could follow him in life forever.
It's a very scary situation, but I hope everything goes right for the three of you. What indications do you have that they're interested in you as you are in them?