by Davo Smith.
Inside your functions add_teachflow_instance and update_teachflow_instance what processing are you doing on the 'stuff' value returned?
You can't save an array of values directly into a database field, you need to do some sort of conversion first (e.g. call 'implode' on it, to convert it into a string). I strongly recommend calling 'var_dump' on the paramaters to the 'update_teachflow_instance' function to see exactly what data is coming from the form.