Weak signal: you only went to class and did OK in them.
Strong signal: you had an internship, or undergrad research experience, or part-time employment as a TA/tutor, or have a completed project to show off, or some kind of non-trivial community/group/club/fraternity leadership.
Really strong signal: you published a paper with someone I know and they recommend you to me.
Absolutely, it's very rare for an undergrad to be on a paper. But that's what makes it such a strong signal: it shows they had the grit and maturity to contribute to a research effort to completion, in a team with people more experienced than they are. In an interview, it gives them something non-trivial to talk about and be proud of. That's very likely a strong junior candidate.
There are a ton of things you might look at for a newly graduated undergrad beyond grades: research and other academic projects, sports teams, editor on a newspaper, etc.
"School" is quite variable.
Weak signal: you only went to class and did OK in them.
Strong signal: you had an internship, or undergrad research experience, or part-time employment as a TA/tutor, or have a completed project to show off, or some kind of non-trivial community/group/club/fraternity leadership.
Really strong signal: you published a paper with someone I know and they recommend you to me.