BackgroundTeachers' and parents' judgements of pupils' cognitive abilities influence pupils' daily learning opportunities and experiences, as these judgements affect the difficulty level of materials and instruction that teachers and parents provide. Over time, these judgements thus significantly shape educational success. However, pupils' characteristics, such as special educational needs (SEN), giftedness and socioeconomic status (SES) can influence and bias judgement accuracy.AimsThe present study aimed to investigate the relation between pupils' cognitive abilities and their teachers' and parents' judgements of these abilities, and potential bias in these judgements related to SEN, giftedness, and SES.SampleThe sample consisted of 1073 primary school pupils from grades 4–6 from 77 classes in 16 schools, and their teachers and parents.MethodsTeachers and parents rated their pupils' cognitive abilities. Pupils completed the COVAT‐3, a cognitive ability test.ResultsMultilevel analyses revealed that parent judgements were significantly higher than teacher judgements, but both informants' judgements were equally strong related to the cognitive ability scores. When controlling for pupils' assessed cognitive abilities, the results revealed small judgement biases: negative for SEN, positive for giftedness, and finally positive for high SES, but only in teachers.ConclusionsOverall, the results indicated that teachers and parents can judge their pupils abilities to a moderate degree, but they also hold judgement biases related to SEN, giftedness and SES. As these biases can affect pupils' opportunities, it is important to increase teachers' and parents' awareness.