Parents' and teachers' beliefs and evaluations of young people are important. Using a feminist institutionalist perspective, and drawing on rich data from one in seven nine-year-old children in Ireland, this paper examines mothers' (who make up the overwhelming majority of primary care-givers) and teachers' perceptions of boys' and girls' mathematics performance. The evidence shows that girls' mathematics performance is underestimated by both relative to boys'. Mother's gender bias was evident among high performing children, at all levels of children's academic self-concept, and among mothers with at least third level education. While the judgements reflect children's actual performance and engagement, a notable gender gap remains. It is suggested that the results reflect gender stereotypes: overestimating boys' and underestimating girls' mathematics achievements. The article indicates the importance of the informal dimension of institutions and the part played by women in the effective devaluation of girls by endorsing gendered stereotypes. Women teachers are less likely to rate children highly in mathematics, taking account of performance: arguably reflecting their own lack of confidence in mathematics assessment. The findings raise concerns for girls' futures since mathematics is seen as an indicator of intelligence. Given the move towards teacher-assessed grading during COVID-19, understanding, and challenging, gender-stereotyping is pressing.