I received the following answer from a flight school in the states :
There has always been a lot of "talk" around about these visas. Of course,
the few flight schools that offer them will tell you that it is absolutely
necessary.
But if you are coming to the US primarily for the purpose of "vacation",
the tourist visa is good for up to 90 days. The key word is "primarily".
So........if coming over, you would state your purpose of the visit is for
vacation only (and not mention anything else).
Immigration's own website will read that a student visa is NOT needed if
you would be in a course of study that is less than 22 hours per week of
classroom OR less than 18 hours per week of "practical" study.