Forgive me for disagreeing, I am right with you on wanting to help people fly in the USA but actually many people enter the USA under the visa waiver scheme or ESTA - without a visa. (for flight training or for vacation or business). I have entered many times without a visa. I have also entered with a visa when required to do so (both O and B1 and B2) and I am clear about the difference. I am from UK, that is generally regarded as foreign
I say this not to be pedantic but because there is already so much contradictory advice out there. There isn't a poster on here who isn't confused in one form or another and thus best to be accurate as possible.
I always take the oportunity when entering the USA to ask questions of the immigration officials on the subject of flight training and have based my choices on their advice as much as anything - they are the ones who choose to admit you after all. I declared my intention to flight train in a recent B1/B2 visa interview at a USA Embassy and was granted it immediately without further query. I have flight trained / done check rides without a visa and declared that on entry and been admitted.
Of course one needs a M1 visa via an I-20 from a SEVIS approved school in some circumstances. Just not all circumstances.