Police officers arrested a second 15-year-old boy on conspiracy to commit murder, after studying social media sites.
They removed the teenager from class for questioning and arrested him, but later released him without charge.
"A 15-year-old male was arrested in connection with the investigation"
West Yorkshire Police said last night in a statement: "A 15-year-old male was arrested in connection with the investigation but has since been released without charge."
Ann Maguire was stabbed to death in front of her Spanish class at Corpus Christi Catholic School in Leeds on Monday.
61-year-old Ann, who had planned to retire this year, suffered more than six knife wounds in the attack, mainly to the neck and back.
During the attack Mrs Maguire fled the classroom before collapsing in a pool of blood in front of another room full of teenagers. She was pronounced dead at hospital.
A 15-year-old boy, who cannot be named for legal reasons, is due to appear by video link at Leeds crown court today.