Some sage advice and some not so sage advice there!!!
The 90 day limit is the period which a UK passport holder is allowed in the US without a visa. Schools that say you dont need a visa are being economical with the truth!! You dont need one since you will probably be let in along with the thousands of others here to see Mickey and Co.
But the INS (Immigration & Naturalisation Sevice) have every right to demand an explanation of what yoy are doing here, how long for, where you are staying and how you are funding it all. One wrong answer and its back to UK!! An M1 visa is no great shakes - it makes it all legal.
You really should be sure to get insurance that specifically covers PPL flying. Almost all travel policies count piloting a light aircraft as a hazardous sport and exlude it from their cover unless you pay teh extra premium. Touch wood you dont, but if you should need medical assistance arising from an aviation incident, you could be out on a limb!
Dont think that I am trying to be a killjoy! I came over here last year without a visa and did some hours building. I had a few questions at the INS and had to be careful with my form filling etc but it was all OK. My advice is, whatever you do, go in with your eyes wide open!!!