The issue that many people are pointing out on this thread is the visa. From what I was informed by the school I was instructing at, an individual would need to be remunerated in order for this to be considered "work". This appears to be a grey area as although there was no payment some might argue accumulating flight time to be remuneration.
It is not a grey area at all. - It is illegal. You were performing uncompensated work without a valid visa.
Of course the school didn't have a problem with it...They were getting gullible labour for free and more profits from the students' tuition.
Many other types of uncompensated activities also require visas. By your definition, the students you were teaching didn't need student visas either?
While I may be able to forgive your ignorance, I have a huge problem with the school. A school which is
authorized to process visas by the immigration authorities have a huge responsibility to stay on the right side of the law. US immigration could come clamping down hard on a school which is proven to abuse the system and I hope they will. At the very least the school's
authorization to process visas and accept international students could be revoked, and in turn wipe out their main source of business.