David Hamlani, already serving time behind bars for other crimes, will now have to wait until the end of November before he is released.
The 17-year-old appeared before South Tyneside Youth Court yesterday in a sentencing exercise that lasted more than five hours, watched by his father Stephen, a detective constable with Sunderland CID.
Magistrates heard the teenager had spent three days with friends downing cider and snorting temazepam, when he found himself staggering towards a stranger in Heathway, Jarrow, at 2pm on Monday, July 10.
Bare-chested and his face encrusted with mucus, Hamlani demanded the 42-year-old woman hand over her car keys, then started threatening her when she refused.
When the terrified woman called out for help, Hamlani grabbed her wrists and struggled with her, then lurched over the road, pulled down his tracksuit bottoms to expose himself and shouted obscenities at his victim, who by this point was in floods of tears.
At South Shields Police Station after his arrest, he spat at an officer and answered interview questions with silence and foul language.
He admitted a public order offence but took a charge of indecent exposure to trial, forcing his victim to give evidence, after which he was found guilty.
Yesterday his solicitor, Christopher Brown, tried to persuade magistrates not to impose a custodial sentence.
He said: "So much more can be achieved in the community. So much more is there to be addressed and encouraged and his ways are there to be changed."
The court heard that Hamlani's criminal record, described as "worrying for a boy his age", includes robbery, weapons offences and violence.
His current spell behind bars was due to finish at the end of next month.
On each charge, magistrates sentenced him concurrently to a four-month detention and training order to start at the end of the sentence he is already serving, meaning he will spend two months in a young offenders' institution and two months under strict supervision.
Two weeks ago, a Gazette application to name Hamlani was granted but we were told it was not in the public interest to publish his address. After another application yesterday, that decision was upheld.

By MURRAY KELSO
murray.kelso@northeast-press.co.uk