You can tell I've been in the business world too long, because these questions wouldn't phase me. 😛
Don't avoid the question, they will pick up on that right away. I also don't believe in lying in any interviews. Tell the truth, but in a very flattering way.
"I have to admit that I have been primarily leaning towards [school A]. It is an attractive option because
[list practical reason here: close to family and home, low tution fees, etc]. But I must say, I am very excited about the possibilty of coming here because
[list a bunch of reasons why you would be super happy there, ideally things that set it apart from you first choice]"
If you're good, you can be completely honest in stating your first choice of schools, but yet you can give the impression that their school is where you'd really *want* to be. And I'm not talking about totally BSing either. Be honest in your praise of their school - you applied there for a reason!
Not that I'm an expert by any means when it comes to school interviews. But I can't imagine it is all that different than a job interview, and that I have a lot of experience (and success) in.