The Cheltenham - Gloucester train journey is about 10 minutes, and frequent.
The number 10 bus stops outside the bus station (a few minutes walk from the train station, very easy), and then wiggles past Ribston and Crypt (there is a number 10 bus stop at Crypt).
You can get a season ticket for the train - ask for a season ticket for the specific dates, you don't have to buy them in particular size chunks (e.g. one month/one week). If you don't buy a season ticket it costs slightly more, £2.20 per day.
You can get flexiriders for the bus parts of the journey: on the bus you buy a flexirider card with 12 slots on it, for the price of 10 journeys (£12) - use one slot for each separate journey.
Or, the number 10 and 94 buses go all the way from Cheltenham to Gloucester - the number 10 is quite a slow journey, though.
ETA: the 07:45 train to Gloucester gets them to school on time; the 07:17 number 10 bus from the promenade gets them there on time.
Thank you so much for this info! Son has been offered Crypt so that's a lot of money each month,
Older sister at Ribston so more than happy for train with her chaperoning... She would hate it!!!
will look into it