Last year someone deliberately sat in my seat in math to bother me. I ended up threatening this person with bodily harm if they did not return my seat. Happily, they ceded it to me and I was able to resume my day, although not without a fairly high stress-level.
No, I can't tolerate it if someone takes my seat unless the entire class is being moved around because we have a seating chart (I'm in high school at the moment). I always sit in the same general area, usually in front or second row and towards the edge of the room. The edge bit is more important than the front row bit to me, so if the only seats left in class are in the middle I will take a seat on the edge in order to feel more comfortable. It makes me very upset if I end up having to sit in the back or near the middle, since in the case of the former I cannot see the board and hear the teacher well (I have quite poor listening comprehension and am nearsighted) and in the case of the latter I am very uncomfortable.
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cogito, ergo sum.
non cogitas, ergo non es.