Do colleges/universities know which high schools are harder than others?

YES - especially high schools within their own state or in neighboring states. Normally, your HS counselor will send information about your school to every college you apply to, which will help even out-of-state colleges better understand your HS background. This is exactly what happened when my kids - who both attended a nationally ranked HS - applied to colleges. Each college received a profile of the school along with the counselor and teacher recommendations.