Three student vs three students: why this costs you Band 7

The single plural -s error in "three student" signals to the examiner that you do not control basic noun agreement. In IELTS, that is a Lexical Resource and Grammatical Range and Accuracy hit at the same time. Check every quantifier: few, several, many, both, a number of — all require a plural noun. Countable nouns must match the number word. This is not a small slip; it is a structural gap that caps your score.