Not very familiar with ColCHS but apparently it is quite a long walk or a slow bus from the station,
The actual train journey direct from Stratford to Colchester is the same as Stratford direct to Sourthend Victoria - c.52 minutes. If you get the 0702 from Stratford to Southend Vic, you can get a bus - Arriva route 29 - at 0812, terminating at the Westcliff Schools about 15 minutes later; there are other buses. Alternatively you can get the c2c line from West Ham to Chalkwell then a 20-25 minute walk.
For SHSG, you can get a bus from Southend Vic - or a few minutes walk to a stop avoiding the loop down to the travel centre and back - or you can get the c2c line from West Ham to Southend East, then a 15 minute walk mainly on the flat, unlike the walk from Chalkwell to WHSG...
BTW c2c is cheaper than Greater Anglia, and has nicer trains
And there are lots of nice places to live down here which are very convenient for both WHSG and SHSG.