We consider the influence that mobile pupils have on the academic achievements of other pupils in English primary schools. We find that immobile pupils in year-groups (a la US "grades") that experience high pupil entry rates progress less well academically between ages 8 and 11 than pupils in low-mobility year groups (grades), even within the same school. The disruptive externalities of mobility are statistically significant, but actually very small in terms of their educational impact. An increase in annual entry rates from 0 to 10% (a 4 standard deviation change) would set the average incumbent pupil back by between 1 and 2 weeks, or about 4% of one standard deviation of the gain in pupil achievement between ages 7 and 11. (Contains 1 figure, 7 tables and 7 footnotes.)