The chat was held to launch a scheme for key stage 3, now open to all secondary schools, in which pupils can talk to members of the Houses of Commons and Lords.
Politicians expecting an easy ride should think again after the grilling Mr Balls received from Ossett School near Wakefield in his West Yorkshire constituency. While warming up for the session, the pupils were asked if a video conference with their MP would make them more likely to vote. "No," came the curt reply.
The representative from the Parliament Education Service pressed on, asking whether the sixth-formers were aware it was possible to organise a free visit to Westminster through their MP. "Yes, but we live in Yorkshire," they responded.
And that is the point of the new scheme: to overcome distance and to bring schools together with politicians, Parliament and each other. It also provides a curriculum resource for citizenship and even history: the first topical video conferences deal with prison reform.
Mr Balls' initial pleasantries were met with silence from the students, who were keen to get on with weightier questions. They opened by asking what significance his friendship with Gordon Brown had on his ministerial role. Mr Balls said Mr Brown treated him exactly like other cabinet members, but ended up admitting: "It is probably true to say that giving me that job showed he thought it was very important."
There was tougher to come. Why was cannabis illegal when tests showed alcohol was much more damaging, asked one boy? Mr Balls replied that cannabis was not something anyone should smoke, before adding, riskily: "Alcohol does more damage if you drink it to excess and damage your liver, or get involved in violence in town centres."
"So everything is ok in moderation?" his teenage inquisitor asked. Mr Balls was too canny for that. "Everything is not ok in moderation, not if against the law," he said.
He was equally cautious when asked if he thought Boris Johnson, the new Conservative London mayor, could improve Labour's prospects in the capital, offering a non-committal: "We will have to wait and see."
A pupil piped up, saying: "London's welcomed him." Mr Balls, a champion Boris-baiter, responded with a guttural "Yeahhhhhhh!"
Schools can book a Parliamentary video conference on prison reform, operating until next Friday, by going to www.janetcollaborate.ac.uk To arrange a video conference with a local MP or peer, email email@example.com or tel 020 7219 4493.