I think the best advice is take your business elsewhere. However, assuming your deposit is non-refundable, you’re probably going to just lump it. The lesson for the future is, as has already been said, do not pay up front, but if you have no alternative do so by credit card.
It is unreasonable practice. The car analogy is not valid as a car is not delivered over a 6 month period. Obviously there is a potential risk to you, but extra cost even if your money is safe. They should go to a bank if they need working capital, not borrow from their customers.
There are lots of rumours about the 2 biggest flight schools being up for sale. If you are referring to one of these, it is even more worrying. However, as both are owned by big corporations I’d think it unlikely that their parent companies would let them go bust even if they failed to find buyers. They would just liquidate them if they were determined to exit the scene.