how important is it to have graduated from a top school? Is it necessary for new hires?

It depends on what position you're after, but a lot of at&t is staffed by "old guard" IBEW/CWA linemen who don't care about your schooling at all and still expect you to either be professionally experienced or else go through some sort of on-the-job apprenticeship. If you're applying for an upper management position they'll care a bit more, but honestly as long as you can demonstrate your ability to do the work your should be fine.

I don’t think too school is necessary but prior experience is required. The best option for new hire is to either come as a contractor first or get some experience with engineering vendors and then apply.

Hard skills are much more important than where you went to school. You do not need a top school background to be hired here; you need skills to do the job.

It is not necessary. Work experience always trumps college in all fields at the company. Several management positions are filled by people who are non degreed.

No, if we use prestigious schools only we miss out on individuals who have worked just as hard to earn their degree.

No