New research from Oxford University Press suggests that the vocabulary of four out of 10 pupils in their first year of secondary school is so limited that their grades are suffering.
Teachers from schools surveyed during the study believe that the issue is getting worse, exacerbated by a drop in the number of children reading for pleasure.
Do you have a better vocabulary than a pre-teen? Take our tricky quiz to find out
Kids are struggling to understand exam papers
There is major concern that children with limited vocabularies will struggle to understand test questions, leaving them unable to complete exams.
It is not just English results that are affected by the so-called 'word gap', but also history, geography and religious studies.
Even if a pupil does understand all of the words in a test paper, a low vocabulary means that their written response is likely to be inadequate and will receive a poor mark.
Home life plays a big part
Circumstance and home environment play a large part in forming a child's vocabulary.
Being encouraged to read and take part in conversations at home will form habits that can help vocabulary skills, but there is little schools can do to influence these factors.