I suppose I’ll start off the questions 
理科教室の黒い影
Kadokawa Page 7 (1)
「そうかい、すまないなあ」I have no idea except すまない is casual すみません. I’m not sure who’s talking either
ぼんやりのようにも見えるし、なにを考えているかわからない、きみ悪さが感じられるときもある。My attempt at translating: Because Kazuo appears to be absentminded, and you don’t know what he’s thinking, -----------. I don’t understand that last clause at all, and I’m not sure of the usage of も in the first one.
Kadokawa Page 8 (2)
吾郎は胸をそらしていった。まっかにふくらんだ顔のため、しょっちゅうりきんでいるように見える。
Attempt: Gorou puffed out his chest. Because of his red face, he often appears to be bluffing.
「芳山くんは、まるでぼくたちを、赤んぼうみたいに思ってるようだぜ。」
This one was tricky but I think it’s: Yoshiyama seems to think that we’re just like babies/children <EDIT: This is the correct translation>
一夫は、夢みるような目つきのまま、ぼんやりとそういって、のろのろと手を洗いつづけた。
Attempt: Kazuo, his eyes remaining dreamy, said so vacantly, and slowly continued washing his hands. <EDIT: As is this>
Thanks in advance d(^^)