Is this plagiarism???

Hm. Yes, I think this would count as plagiarism. Something as well known as 'It' has quite a definite narrative. However, I doubt you'd be able to get much of it into your short exam response. I remember a similar situation a few (more than a few) years ago when a student of mine submitted a piece of original writing coursework that was based around the plot of 'Gothika'. It was just too obvious and the fluency of the narrative was affected. I think it's best to go with your own ideas. You'll feel better for it. Even if the connections to the original are tenuous, you still run the risk of raising concern with your examiner.

The most amusing occasion I've managed that I've been reminded of here was when a student submitted 'The Little Match Girl' by Hans Christian Anderson, directly copied and pasted from the internet in full and florid Victorian prose. They had the audacity to claim it as their own work until I brought the whole thing, and YouTube video, up on the whiteboard. Very entertaining.

The writing examination is for you to show your skills and your creativity. Every text is influenced by something else, sure, but don't make it too derivative. That's my advice.

Yes, exactly. After all, there are lots of similarities between say 'Alice in Wonderland', the Narnia books and then 'Coraline' but they're all very different stories.