For a differing point of view...
I would suggest dressing business casual. I definitely subscribe to the school of thought that being overdressed is better than being underdressed. However, you are applying for a CFI position. If this was a career position I would fully advocate dressing the part. However, since in this career, this job is treated merely as a temporary position, a tie may be a bit overkill. In my opinion, that is what the market based wages, which are low, and quick turn overturn-rate suggest. Hope the interview goes well.