We do not hold data on the number of schools that teach computing at Key Stage 3.
Computing is compulsory at Key Stage 3 in state maintained schools. Academies and free schools do not have to follow the National Curriculum, but may still offer computing as part of their own curriculum and may use the computing curriculum as a benchmark.
We do hold Key Stage 4 data. In 2016, 62,703 pupils entered the Computer studies/Computing in GCSE, with entries in 2,405 schools.
 https://www.gov.uk/government/statistics/revised-gcse-and-equivalent-results-in-england-2015-to-2016 (Subject time series table).
 Based on Key stage 4 qualification and subject data, available at: https://www.compare-school-performance.service.gov.uk/download-data.