You should not talk negatively about your current employer; and do not tell that you want a new job because you need more money.
Here is a good possible answer to the above question:
I am looking for new challenges in a prestigious school. During my discussions with one of my colleagues I got the impression that you are trying to create new programs and courses and you promote modern methods of teaching based on modern software. I got the impression that your school is very dynamic and open to innovations, especially in the field of ESL teaching.
You will find more ESL job interview questions with provided answers here...