Can I report a rumour to the police?

NSFW just in case

For some context, theres a rumour going around in my year of someone having created and owning cp of their sibling. His sibling is underaged and he is 18. It is most likely true.

I was disgusted when I heard of this but I don’t know whether I should report him over a rumour. I don’t know if whether teachers will do anything, or whether they will even report him to the police for image sake (there are important public events coming up and I know people have gotten away with things like rape in the past because they reported it to teachers instead of the police)

What should I do?