I would disagree with this. It might be bad form, kindof, but the school can probably get some money back from canceling the flight (or may be able to change the flight to accommodate another student). If said person went, they would not only be wasting the schools time but also their own time. It may be an inconvenience, but if you know you're not going to that school, why waste their time when they could be spending more time with the students that have agreed to go or perhaps with another student?
If going to an interview is going to cause you to miss more school/work, then this is especially important. You're probably missing a lot of class anyway. I'll be gone for nearly 40% of my quarter, which is going to be really hard. If I knew I wasn't interested in the school, even if it is a little bit of bad form, I would just explain that I can't miss any more school.