17 year old arrested without charge

[Suetheramble]

My 17 year old daughter was arrested because she was in the company of a boy who knocked a brick from a wall as they walked passed. The police were called and all 4 were arrested. My daughter told the truth but refused a solicitor because she said as she had not done anything wrong she did not think she would need one. She refused a phone call because she thought we would go mad ( not wrong there!) This boy took 12 hours to admit the crime and my daughter was released without charge after being in a cell for 12 hours.
My query is;- Why have I and my daughter had a letter from the local authority "refferal and assessment service" saying this was a criminal offence and daughter needs to understand the seriousness of her act and hopefully this is a one of offence but if she continues she could end up with a criminal record.
The government procedure that this information has to be stored in the system.
Is she not innocent until proven guilty and why are these records held?