I have two supervisors, in different countries, one in home institute who sent me as an exchange student to the host institute, where I closely worked with another co-supervisor who significantly contributed to my work, taking place of the last author in the conference paper I have submitted (not published). He reviewed my work, my paper and also was a really good advisor. My other supervisor, one from the home country had nothing to do with my project except on record. His domain is completely different from what I have worked on.
Now, in my master's thesis that I presented in the home institute in front of my co-supervisor and other faculty in the institute, I mentioned that I've submitted a paper based on the work I had done. I was questioned about the authorship of this co-supervisor, and was told it is unethical to leave him out of authorship, because by default, the supervisor must get the last authorship.
Now I fear that due to ego issues, they will reduce my grades because I didn't mention this co-supervisor's name in the paper.
How do you suggest I tackle this situation and am I wrong to leave him out? His only contribution was to provide me recommendation for sending me to the exchange program, and review my final thesis and presentation which I suppose every supervisor must do, whether or not a paper is published.
P.S. In my institute, having a paper published gets us better grades and hence I decided to mention about the submission. But now it looks like everything is going to take a U-turn on me.