42And when he was twelve years old, they went up to Jerusalem after the custom of the feast. 43And when they had fulfilled the days, as they returned, the child Jesus tarried behind in Jerusalem; and Joseph and his mother knew not of it.
46And it came to pass, that after three days they found him in the temple, sitting in the midst of the doctors, both hearing them, and asking them questions. 47And all that heard him were astonished at his understanding and answers.
-- Luke 2:42-43, 45-47 (KJV)
Does anyone imagine that during the remainder of Jesus' youth and early adulthood he wasn't a frequent visitor to the Temple, that he didn't continue asking the doctors questions, learning from them whatever was needed "to know his letters", i.e. to be literate?
Matthew says this:
54And when he was come into his own country, he taught them in their synagogue, insomuch that they were astonished, and said, Whence hath this man this wisdom, and these mighty works? 55Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
-- Matthew 13:54-55 (KJV)
Jesus continued to astound in regard to the depth of his knowledge and understanding. Now, it might be forgivable for some of Jesus' own town to be unaware of what he did during his visits to Jerusalem, but it is not forgivable for those of them who attended the synagogue on the Sabbath, because Luke says this:
16And he came to Nazareth, where he had been brought up: and, as his custom was, he went into the synagogue on the sabbath day, and stood up for to read. 17And there was delivered unto him the book of the prophet Esaias. And when he had opened the book, he found the place where it was written, ...
It is clear that Jesus was literate, and was familiar enough with the text of the scriptures to immediately look up a relevant passage when the need arose.
Matthew records Jesus saying to his listeners:
"Have ye not read what David did ..." (Matthew 12:3)
"... have ye not read in the law ... (Matthew 12:5)
"Have ye not read, that he which made them at the beginning made them male and female," (Matthew 19:4)
"... have ye never read, Out of the mouth of babes and sucklings thou hast perfected praise?" (Matthew 21:16)
"Did ye never read in the scriptures ..." (Matthew 21:42)
"But as touching the resurrection of the dead, have ye not read ..." (Matthew 22:31)
Jesus' own words declare him to be a reader, so people are without excuse for continuing to believe that Jesus was simply an illiterate carpenter.
Those who claim to be literate themselves, have you never read about the time Jesus stood up in the synagogue to read?
The OP asks, "Does John 7:15 mean Jesus was untaught?". The answer is no. John 7:15 would be a typical resort for those who might be intimidated by someone of lower station having a better grasp of the scriptures than they do -- playing the man, not the ball (so to speak).