Ella thought she was being kept in at break for misbehaviour. In fact, her crime was much simpler: she is a girl.
Ella, 4, was not tired during naptime, so she spent it lying on her mat playing with toys. Meanwhile, her classmates, Andy and Jon, ran around talking, reading and drawing.
Afterwards, the boys were allowed out to play, while Ella was kept in for her consistent refusal to go to sleep.
Such double standards are not unusual: teachers everywhere regularly operate one set of rules for girls and another for boys.
Carrie Paechter, who researches gender and education at Goldsmiths College in London, believes this divide is deeply ingrained. For example, many school uniforms encourage stereotypically female dress. "This reinforces particular ways of being female," she says. "And that affects how girls see themselves."
These attitudes are compounded by teachers, who anticipate different behaviour from boys and girls. Ideals of femininity are invoked from nursery onwards. "Girls are asked to sit nicely, so their knickers don't show," says Professor Paechter. "There's still a lot of attention to girls' ladylikeness."
Often teachers will pick girls up on "unladylike" behaviour. "Boys are expected to be boisterous," Professor Paechter said. "Girls who are challenging, or who push other kids around a bit, the way many boys do, are taken much more seriously."
At secondary, teachers who speak idealistically of boosting girls' confidence nonetheless associate female assertiveness with arrogance.
And the line of acceptable behaviour is drawn in different places for girls and boys, according to Carolyn Jackson, a psychologist at Lancaster University. "Minimising disruption is always an issue, but teachers expect more interruptions from boys and are more tolerant of them," she says.
"Some of the ways in which boys disrupt lessons are seen as harmless fun. But similar behaviour by girls is usually regarded very differently."
Teenage boys who smoke, swear, fight occasionally, and are cheeky, are often seen as exhibiting the same high spirits as Ella's naptime classmates. But when adopted by girls, this behaviour is labelled "ladette".
Laddish behaviour is problematic, but discussions tend to focus on lads' academic achievement. Concern about ladettes, however, focuses on sexual behaviour and their safety. "Femininity remains a moral condition," says Dr Jackson. "Teachers find drinking much more worrying when the drinkers are girls. The same applies to fighting."
Ella's case was originally noted by Jennifer James, of Kent State University in the US. She found that girls who suffer such unfair treatment early on are less likely to feel successful, and more likely to bully, play truant, or fall pregnant as teenagers.
Such unwitting sexism can be hard to combat. "Teachers don't set out to produce sexist classrooms," says Professor Paechter. "But it's difficult to be objective or think before you act."