How did Jesus come to speak in the synogogue (Luke 4) when He had never attended Rabbinical Schools?

John 7:15 shows us it was public knowledge that Jesus had not learned letters. This was their way of saying that they knew Jesus had not attended the Rabbinical Schools in order to be qualified as a Rabbi- and thus to formally teach. Can any one declare how someone deemed unqualified would be given such permission to teach in a synogogue?