This thread has been done before. You must get an M1 visa for training. If you enter on a waiver you are asking for trouble. The school you are going to in Lancaster will submit your details including your fingerprints and photograph to the TSA. Those details include your visa information. No visa - arrest and deportation follow quickly afterwards.
Any school that tells you you can be trained on a waiver is to be avoided. There is a lot of information on the TSA web site and the SEVIS site about this. See
http://www.unitedstatesvisas.gov/
When you get to Lancaster visit the helicopter school next door. It is owned by a Brit with Brit instructors.
Chopperpilot 47