“And will it really come by owl?” Lily whispered.
“Normally,” said Snape. “But you’re Muggle-born, so someone from the school will have to come and explain to your parents.”
Deathly Hallows, Chapter 33: The Prince's Tale
Harry, claro, é um caso especial, com sua história e tudo mais. Dumbledore suspeitou que Dusrleys deu problemas e, portanto, ele manda o Hagrid explicar pessoalmente as coisas para o Harry. Dumbledore confiava em Hagrid para fazer isso, visto que ele também era o único que tinha sido enviado para pegar o bebê Harry de Godrics Hollow.
"Sorry?" barked Hagrid, turning to stare at the Dursleys, who shrank back into the shadows. "It's them as should be sorry! I knew yeh weren't gettin' yer letters but I never thought yeh wouldn't even know abou' Hogwarts, fer cryin' out loud! Did yeh never wonder where yet parents learned it all?"
"I never expected this," he said, in a low, worried voice. "I had no idea, when Dumbledore told me there might be trouble gettin' hold of yeh, how much yeh didn't know. Ah, Harry, I don' know if I'm the right person ter tell yeh -- but someone's gotta -- yeh can't go off ter Hogwarts not knowin'."
Philosopher's Stone, Chapter 4: The keeper of the keys
P.S: Edit sugerido por @Vishvesh
O próprio Dumbledore foi recrutar Tom Riddle, já que ele não tinha pais e alguém precisava explicar as coisas para a cabeça do orfanato.