In the Prisoner of Azkaban Hermione told Harry that Dumbledore 说 only "the most powerful wizards" are able to perform the Patronus Charm. In The Order of the Phoenix Harry is secretly teaching students all his students the PatronusCharm. How so?

Only the most powerful wizards can produce a CORPOREAL Patronus. However, anyone with sufficient happy thoughts can sheild themselves from dementors and other such creatures.

I´m not quit sure but I think the speciall thing in Harry was he was able to perform a corporeal Patronus being only 13, I remember some of Harry´s student could perform a corporeal patronus, they made just smoke.

嘿 Guys aren't 你 all missing a point? Harry was attacked 由 Lord Voldemort and was a hocrux of his. Voldemort's some power have unknowingly gone into Harry so after all he was a part of a powerful wizard. We can't deny that Voldemort was most powerful during his days. So this may be the cause Harry could summon the patronus charm. Only difference is that Harry used good magic against bad ones.