Did Jack and some of his savages came to the shelters on the beach and steal the conch in the Lord of the Flies?

No, Jack along with Roger and Maurice raided the shelters at night and stole Piggy's glasses, so that they could light fires at castle rock. The conch was destroyed, and Piggy was killed, when Roger toppled a boulder from the top of castle rock, when Piggy came to demand the return of his glasses.