Page 4: "I thought I explained about the misunderstanding during homeroom didn't I?" - I think this is fine, although my brain auto-corrected it to "I thought I explained the misunderstanding during homeroom didn't I?", and when I re-read it I thought that maybe the part saying "I explained about the" was torn between saying "I explained the" and "I talked about the".
Page 4: "The one who remarried is my father and Iijima's mother" - This is mostly correct, but it would be better as "The ones who remarried are my father and Iijima's mother". My reasoning for this is that in this situation using "ones" instead of "one" is due to there being multiple subjects, and saying "who remarried are" instead of "who remarried is" was because of the sentence structure - if using "is" and "one" in this way, it would have to be laid out as "The one who remarried is my father, with Iijima's mother".
Page 4: "It's the one day a female classmate become my sister!!!" - This bubble is fine as it is since it still gets the point across, but to look neater it could be written as - "It's the "One day a female classmate became my sister!!!"" (become in the sentence can instead be became or becomes, since became and becomes is more appropriate in the sense that "someone becomes something" or "someone became something" instead of "someone become something")
Page 19: "Why not tell everyone about your 'at home only girlfriend'?" - another part I had to reread because my brain auto-corrected it. I read this as "Why not tell everyone about you being an 'at home only girlfriend'?" which I guess is the intended message, but shortened, so if sentence length is the issue I'd say that this would be better; "Why not tell everyone you're an 'at home only girlfriend'?" as it gets across the same message in the intended length.
Page 20: "Then, that's the more reason you should make the first move!!" - Pretty much fine, another instance of subconscious auto-correcting. The "the" in the sentence seems out of place when reading it normally unless you put "all" before it, like so; "Then, that's all the more reason you should make the first move!!" as just "the more reason" / "the reason" implies a single reason (while still worded a bit strangely), whereas "all the more reason" implies an "even better reason or cause" for something, in the story's case for conveying her message.
Page 28: "Don't you feel disgusted when someone not close to you pat your head?" - In this case, the word "pat" should be "pats" (pat would be "to pat, e.g. Bryan gave him a light pat on the arm", pats would be "someone pats something, e.g. Bryan pats him lightly on the arm", so it's a give vs gives situation).