Sunset's memories of high school were erased. Not just Mane 6, Canterlot High, but all her memories as a higher-level pupil of Princess Celestia as well.

Also, humans don’t destroy weapons, they secure and contain them. Sunset knows to destroy the memory stone as per Princess Twilight’s message.

Oh wait, knew.

The human mane 6 don't remember much either. Their memories are fragmented. It was Princess Twilight, then Sunset, who united them. It was the magic ponies who knew how to deal with magic threats. Not humans with magic.

What a pickle.