St Dunderhead's

St. Dunderhead’s School at Fogwell has a high reputation for hockey – but not so high a reputation for veracity. The First XI played a match at Diddleham recently, after which the girls were allowed to go to a concert. Miss Pry, the mistress in charge, collected the team afterwards; she saw ten girls emerge from the concert hall and one from the cinema next door. When she asked who had been to the cinema, the members of the team replied as follows:

Joan Juggins: ‘It was Joan Twigg.’
Gertie Gass: ‘It was I.’
Bessie Blunt: ‘Gertie Gass is a liar.’
Sally Sharp: ‘Gertie Gass is a liar, and so is Joan Juggins.’
Mary Smith: ‘It was Bessie Blunt.’
Dorothy Smith: ‘It was neither Bessie nor I.’
Kitty Smith: ‘It wasn’t any of us Smith girls.’
Joan Twigg: ‘It was either Bessie Blunt or Sally Sharp.’
Joan Forsyte: ‘Both of the other Joans are telling lies.’
Laura Lamb: ‘Only one of the Smith girls is telling the truth.’
Flora Flummery: ‘No, two of the Smith girls are telling the truth.’

Given that, of these eleven assertions, at least seven are untrue, who went to the cinema?

(Source: Can You Solve My Problems? A Casebook of Ingenious, Perplexing and Totally Satisfying Puzzles by Alex Bellos)

It isn't necessary to go through every possibility. After evaluating the statements for the first two girls I noticed a lot of the statements have to do with the Smith girls, so a lot would hinge on one of them having gone to the cinema. And in fact it was Dorothy. Checking this is trivial, so I will omit a full solution.