Assuming the 175 to be right, 30 of those would presumably be CRGS 's sixth form borders. That leaves 145, so approx 11 per year group at each school, which is only about 10%.
If you want to see a 12.5 mile radius applied to the centre of colchester then this tool is good:http://www.distance-calculator.co.uk/towns-within-a-radius-of.php?t=Colchester&c=Great%20Britain
Unfortunately it defaults back to 25 miles when I post! It doesn't show the suffolk border but as a rough guide follow the river stour.
What I find very interesting is the number of essex towns, particularly in tendring, that would be outside the 12.5 mile limit, so perhaps this is why the Colchester schools havent followed Chelmsford. DD has several girls in her class from Harwich.