Do universities pay attention to the high school you went to?

Yes definitely. There are now so many students getting great grades, it is more likely that the uni will choose not only the one with the best grades but also the person who has done the most extra curricular work and of course better schools often have more money so they offer more opportunities to their students. If you go to a not so good school, you could also do some voluntary work helping younger students or something totally outside of school to show you have initiative and good people skills