In calling Athens "the school of Hellas" in his funerary speech after the first battles of the Peloponnesian Wars, Pericles was saying that Athens was the school of all of Greece. Hellas means "Greek," and is how the people of Greece refer to their country.
After several passages where he praises Athens and its people in his oration, Pericles says, "In short, I say that as a city we are the school of Hellas..."
Athenian society of the time (roughly 450 to 399 BCE) valued education for boys highly, and their reputation for a high standard of education made the comparison a natural one. Most boys, even from poor families, went to school from ages seven to 14 and learned grammar, music and gymnastics.