After two months of secretly messaging Johnson, she finally told her parents, who took her to report the player to the police.
The court also heard today from a friend of the victim, who told the trial she thought the footballer was "a bit of a paedophile".
The teenager told the jury she did not at first believe her friend's claims of a sexual encounter with the ex-Sunderland star.
The witness said: "At first I thought it was definitely not Adam Johnson... but then after I believed her when my other friend told me about seeing his car."
When asked why the witness didn't think it was Johnson texting her friend, she said: "Because he's a bit of a paedophile if he's going for kids at 15, and that."
The witness agreed that the messages between her friend and Johnson could be described as "flirting", adding: "He must have known she was not 16. He had said 'Oh well, when are you going to be 16?' And he said 'Oh, that's ages away'."
The court heard that Johnson and the girl began communicating on Facebook, Instagram and WhatsApp, and the footballer allegedly set up a second Snapchat account so his girlfriend would not discover his activities.
But after the girl showed her friends the messages and discussed it with them, the news spread like wildfire on Facebook and around her school.
It was also claimed today that teachers at the alleged victim's school knew of rumours she was meeting the footballer by the time it was reported to police by the girl's parents.
The friend of the teenager said: "The teachers said if anyone spoke about it we would get sent out from lessons but there was nothing they could do about it because everyone was talking about it."
Another friend of the schoolgirl also gave evidence today, claiming Johnson and the victim had arranged a third meeting.
"She told us that she was going to have sex and sleep with him in his car," the witness told the jury.
But when the girl arrived, Johnson was not there and he messaged her to say he could not meet her because he did not have an excuse to leave the house and girlfriend, Stacey Flounders, the court heard.
"She was mad at the fact that he had messed her about," the witness said. "She had come over to see Adam and then he did not show up. That was the same night she told her mam and dad."
The victim sent a photo from outside Johnson's home, captioned 'Bae's house', the court heard. 'Bae' is text slang for 'before anyone else' and is often used to mean boyfriend or girlfriend.
The jury heard that, on that night, boys in the girls' year at school found out and the subject was discussed in a group chat on Facebook.
The witness said her friend was "pretty distraught" and crying because the news was out.
Over the last week, Bradford Crown Court has heard Johnson was befriended on Facebook by his alleged victim, an avid Sunderland fan, before he arranged to meet her to give her two signed football shirts.
After the first meeting, the pair continued to send each other messages, during which Johnson asked for a "thank you kiss" for the shirts, the jury were told.
When they met for a second time, the girl alleges that she performed a sex act on the football player and he put his hand in her trousers.
Johnson has admitted grooming the girl and kissing her, but denies two more serious counts of sexual activity with a child.
The prosecution say that in messages sent between the two after their meetings, he told the girl he "just wanted to get your jeans off, LOL". He later search "legal age of consent" on the internet.
Johnson is also said to have sent messages to the alleged victim in the same minute as he was messaging his girlfriend, Stacey Flounders, who gave birth to his daughter around the time of his liaisons with the alleged victim.
In emotional evidence to the court, the alleged victim told the jury she now "absolutely hates" Johnson after he "made her out to be a liar".
She only made the more serious sex claims against Johnson in a second police interview, and did not mention them in her first meeting with officers. She said she was trying to protect the star.
During evidence yesterday, friends of the alleged victim told the court she is "someone who can exaggerate" and had taken pictures outside player's home after he pulled out of a third meeting.
The court also heard that the girl had posted messages on a social media account of another Sunderland star, Connor Wickham, who passed them on to police.
Johnson's girlfriend, Stacey Flounders, was not with the star as he arrived at court today. He was accompanied by his father, Dave Johnson.
Where to get help:
•Lifeline: 0800 543 354 (available 24/7)
•Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
•Youth services: (06) 3555 906 (Palmerston North and Levin)
•Youthline: 0800 376 633
•Kidsline: 0800 543 754 (available 24/7)
•Whatsup: 0800 942 8787 (1pm to 11pm)
•Depression helpline: 0800 111 757 (available 24/7)
•Samaritans: 0800 726 666 (available 24/7)
If it is an emergency and you feel like you or someone else is at risk, call 111.
