Key Stage 3 refers to the first 3 years of secondary school, covering ages 11-14 and school years 7-9.
Key Stage 3 is a vital period in your child’s education, equipping them with the knowledge and skills needed to tackle GCSEs in Key Stage 4. During Key Stage 3, your child will take a more specified approach to subject areas, continuing with core subjects from Key Stage 2 while also starting to explore topics such as sex, relationships and careers. End of year tests usually take place throughout Key Stage 3 for each subject and you should receive report cards to help keep track of your child’s progress.
Source Reference: What are the Key Stages in the National Curriculum? - From WHSmith