Classmate is being a creep

I'm male and so is my creep classmate let's call him C, today I heard some other boys in my year (yr 10) talking about how C likes to walk up the stairs behind a certain girl, J. As a guy, this is really disgusting to hear especially because J doesn't know about what C is doing and I also happen to have a crush on J.

I'm considering telling J about what C is doing so she can avoid walking in front of him up the stairs (our school has heaps if stairs) as I would probably want to know if I was J. All I really need is just advice, especially from females for what is should do, the best thing is probably report it right? Please give me advice. Thank you.