[Fixed]-My form with a ModelMultipleChoiceField is not saving data.

Only fields named in the fields tuple are saved to the instance. You don’t have your m2m field listed there.

You also define your modelchoicefield with a different name – class_persons instead of persons. In fact, there is no reason to define that field separately at all – you haven’t changed any of the attributes from the defaults.

And once you’ve removed that definition, there ​is also no reason to override __init__, seeing as you never pass the user parameter nor do you use it anywhere in the form.