A minor is estopped from asserting error after stipulating to a procedure in excess of the court’s jurisdiction. Here the minor was arrested for possession of marijuana. He filed a motion to suppress evidence which was denied. He then admitted the charge in exchange for being placed on informal supervision. (Welf. & Inst. Code, sec. 654.2.) At a later review hearing, the court found the minor had violated conditions of his informal supervision, sustained the petition, declared the minor a ward of the could and placed him on formal probation. The minor filed a notice of appeal challenging the court’s jurisdiction to revoke probation. Although the People conceded error, the court here disagrees. Section 654.2 is to be implemented before adjudication of the charges alleged in a petition. Here the juvenile court adjudicated the petition when the minor admitted the charge. Consequently, the court exceeded its jurisdiction when placing him on informal probation utilizing section 654.2. The minor received the benefit of that error and prejudice resulted not from the irregularity of deviating from the statute but from his own failure to succeed on informal probation. Permitting the minor to challenge these procedures now would be allowing him to “trifle with the court.”