First of all - the reporting is off on this, and secondly the juvenile justice system in the US is significantly different from that of the UK, where anyone over the age of criminal responsibility (10) is tried in a normal court.
In most states, you are tried in a juvenile court up to the age of 18 (the age does vary a bit). Sometimes if the case is serious enough it can be heard in an adult criminal court, but that did not happen in this case - it was in a juvenile court (hence no jury). In juvenile courts, the proceedings are civil, not criminal, but the court can have broad powers over the offender if they are found delinquent (guilty). So the sentencing guidelines are different because it is not an adult criminal court - rape is a category 1 felony in adult court, but is a lower category offence in a juvenile court. And the sentence was not one year, it was one year and two years minimum, maximum until they are both 21 - which is what they will probably get. That decision is up to the state youth justice service. And it looks as though further charges may be brought against more people.
I do wonder whether the State went for a non-jury trial in a juvenile court on the basis they were more likely to get a conviction than with a jury trial in the adult system.