Aggregators like Hacker News and Reddit don‘t copy content as a whole, they just provide links. Providing a preview of the content, as Facebook does, might be considered either fair use or citation—but in neither case they’re copying and republishing the entire content.
Attribution does not magically remove copyright.
If I'm not mistaken, Google got sued for providing short excerpts of news from various news outlets and decided to settle out of court.
The schools position is analogous to claiming that someone is pirating your freely downloadable PDF document because they used Nitro instead of Adobe Reader to view it.