The victim was too scared to call the police in case Knight overheard, so she shut herself in the bathroom and took several pictures of her tearful face on her phone.
Amy's horrified friends called the police and Knight - who has no criminal record and has recently been offered a university place - was arrested.
He was later convicted at a Newcastle-upon-Tyne court of having a BB gun with the intention of causing fear of violence.
Speaking after the court case, Amy, who is an apprentice at a school for children with special needs in Hebburn, South Tyneside, said: "I had no idea where this had come from.
"I was absolutely terrified, I thought he was going to kill me. "He just told me to get in the bathroom because he was getting agitated and saying he was going to hurt me. "When he was banging on the door it was so scary. I even thought about trying to get out of the window but it was too high.
"I told my three closest friends what was happening on Facebook chat.
"I was scared to ring the police in case Josh heard and tried to smash his way into the bathroom.
"I was crying and my fingers were shaking so much that I couldn't type and my messages weren't making any sense.