I have the following 2 sentences:
1- I did not see any other classmates, except/but Michael.
2- I did not see any classmates, except/but Michael
For the first sentence can we understand that the word other converts the expression in inclusive meaning that Michael is a classmate too?
For the second sentence, since the word other is missing, then does it mean that Michael is not a classmate?
Thanks in advance for any help.
In both sentences, the word except (and also but, when used like that) implies that Michael is one of the classmates. If Michael were not a classmate, you’d need to say something like
 I did not see any of my classmates, but I did see Michael.
Having said that, in both of your sentences there should be a determiner in fromt of classmeates. There are in fact many ways how they can be rewritten in order to be fully acceptable. Here are some, which all mean more or less the same thing:
 a. I did not see any other of my classmates except Michael.
b. I did not see any of my other classmates except Michael.
c. I did not see any other classmates except Michael.
d. I did not see any of my classmates except Michael.
e. I did not see any classmates except Michael.
In all of these, instead of except you could use but. Also, in all of them, you could say [except]/[but] for Michael. In those which have my classmates, you could use some other determiner except my; for example, you could say the classmates. The placement of a comma after classmates is also a matter of style.