You will have no limit to travelling in and out of the USA on a M1 visa, just make sure your school signs your I-20 form to allow you to return to the USA (this is of course when it comes time to leave the USA during your training).
There are basically 2 schools doing the ATPL Theory in the USA , that is us at NAC Groundschool, in Naples FL and Atlantic Flight Training/Delta Connection Academy in Orlando FL . I am assuming you will be completing fixed wing training. If you are piloting a flying palm tree then there is Helicopter Adventures in Titusville.
Only students from these 3 schools have open, free access to the Orlando exam center. You may use it if you choose another school but there is a $US400 per student per sitting charge to use the facility if space is available.
Use the CAA list of approved schools to contact any of the above or contact any UK based school.
CLICK HERE TO GO TO THE LIST