Who was right in this situation?

So we had 2 lectures with a little break in between them, the second lecture is given by a very strict and impatient professor so I was in a rush to go to the toilet and back. During the first lecture I was sitting with my groupmates but a few hadn't come for the first lecture. When I came back my seat was taken by a groupmate who just came for the second lecture only, me and some others told him I was sitting there and he just said I should have left the bag for him to know that and refused to go away, so as the auditorium was already packed I had to go and sit on the cold stairs while already feeling a bit sick since the morning.

I feel like technically I was at fault for not marking the seat as mine somehow but still it was just plain rude of him to push out his groupmate like that.
Who was right in this situation?
He was right
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You were right
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Who was right in this situation?
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