The sentence has a few mistakes and so the meaning is not 100% clear. My guess is this: "It turns out that it was a student who set fire to X School". If the speaker had found out the identity of the arsonist only recently, he would say "turns out".
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park sang joonI think if A thinks readers, listeners, or listener knows what he says, he should use "it turns out," and if not, he should use "it turned out."No. Not at all. The listener may not know what "turns out", but the speaker can still say "It turns out that".