She paints enormous pictures, mostly of groups of people in social situations such as having a meal or on a journey. Her influences range from the expressive early 20th-century artist Sir Stanley Spencer to Italian figurative painters such as Caravaggio.
"Once I started GCSE and began to paint, the figures just got larger," Jo explains. "I like them to be as near life size as possible, because it makes it easier for me to relate to them. It's almost as if I'm in the painting myself."
She says of Dartford's head of art, Peter Daniel: "He's been so enthusiastic and encouraging, always getting me to take ideas further, to look at the work of different artists. But he's also given me a lot of valuable practical criticism."
Jo soon began painting at home, at first in the garage, then in her bedroom, which she converted into a studio. She once stayed away from school for three weeks to work on a painting - the school thought she had left. This summer she got a grade A in A-level art, which she completed in a year.
Teachers have encouraged her to get involved with local artists and to attend life-drawing classes. Though she's keen to go to art college and become a painter, she'd like to get a degree first, perhaps in English. "It would help me to express myself about my art," she says.
"Other girls are as talented as Jo," says Mr Daniel. "it's her obsessive commitment that's rare."