I have a DD who is at ColCHS and we live in Witham... There are a number of children who live in Witham and travel to ColCHS or CRGS. DD has always travelled by bus (one on CSSE transport website link) and leaves about 07:50 and returns about 4:30.
I am not sure whether many travel by train because it seems a bit of a hike from the station.
As far as the town goes it is a major commuter town, there are nice bits to live in and not so nice bits.. And some outstanding primary schools..
PM me if you want more detail.
I have pm you.