Pipergirl,
As I understand it you will be tied to just one school with an M1 visa.
The M1 visa will only be valid at the school you name when applying for the visa with the US Embassy. I think any training/study towards a rating or qualification at another school will be in breach of your visa conditions. You can, however, undertake hour building or pleasure flying at any school, if it is not training undertaken in order to gain a qualification or rating in terms of flying.
Please don’t take this as gospel, it’s just how I understand it as I am looking in to finishing my PPL in the US, then maybe some hour building at a different school in the same visit.
Hope I’ve been of some help.
Simkin.