1. First year, in senior high school (During our first year of high school), è There will be “Second year,” “Third year,” in the rest of story. If the story was separated by conventional “during…”, it would removed poetic lustre of the story.
2. by sitting side by side (from sitting side by side, just don’t like two “by”s so close to each other) è If use “from”, that would change the story, because “they” took a break from the game, not from sitting.
3. Our relation of the time (our relationship at that time) è Relation can be used both on people and things. Relationship is rarer now, it is a terrible tongue twister. I don’t see the benefits of “at that time” over “of the time”. You have to convince me on this one.
4. boy friend (boyfriend) è Yes, should be boyfriend
5. repeating the feeling (“repeating the feeling” sounds a little bit strange to me, l like the recurring feeling better) è Using “repeat” indicates the feeling was not real – you have to fake it and then, “I” started to believe “my” own creation; however, feeling could “occur” and “recur” on its own, without “my” control, which does not quite fit here.
6. Out of many her (her many) other attractions è still awkward, better delete “her” altogether.
7. a young man of (an) ethnic minority è I don’t think “an” should be there, immediately after “of”.
8. floating (slipping) è You have to gave me a reason to change that. My rationale is that “floating” is more visual. “I” was pretending horse trotting. Imagine a light foam horse head jumps up and down with my moves. “Slipping” just doesn’t have that comical effect.
9. hat (had) è Yes, typo identified.
10. which (that, or which,) è “which” and “that” are interchange in this case. If you want to use a comma, it should be inserted before “which”, not after. Depending on writing style, you may decide to give the readers a “pause” or not, and in many cases, "which" does not necessarily couples with a comma.
11. it to (onto) è Both are fine, no ambiguity would arise from using either of them. I would like to hear your reason on prefer “onto” to “to.
12. words, (delete “,”) è You are right, this comma is not necessary.
13. couldn’t help talking (couldn’t help but talking), è You have to give me a reason why you want a change.