Yes, you are correct the majority of boys do bus to CRGS. The bus services fall into several catorgies: there are contract services organised via the CSSE, public buses and private bus/minbus services organised locally.
CSSE details http://www.csse.org.uk/transport.htm
Public buses serving CRGS include first buses, network colchester, chambers and hedingham coaches. Prices vary a lot depending on route, but as a guide a child weekly inner colchester zone ticket is currently £12.
From Suffolk there seems to be two options, some boys train into Colchester from Ipswich or manningtree and then bus up the hill, or there are some mini bus services. The later is probably quicker but the train would allow after school club attendance easier.
With regards to free transport, very few are eligible, you must live 2 miles away and be eligible for income support, or live 3 miles away and not have a closer catchment school ( due to the location of CRGS I am not sure that this is possible!)