Both modern and ancient Greece are referred to as "Hellas." Athens was renowned for the many schools it had. These schools were comprised primarily of philosophers and their students. Pericles is referencing these schools and is in essence referring to Athens as "the school of Greece."
The actual ref to school of Athens was at the funeral service (so how we commemorate those lost in our 2 world wars for example), pericles oration to athens in his campaign for war after fighting the peloponnesian war against Sparta for only a year. The phrase ref to athens being not an imitator but rather a school to the rest of the world to learn from. So he refers to democratic athens and all her achievements which by this time the Parthanon had been built also equal justice under the law.