I doubt they will mix TR'd with non TR'd for ground school, the short course on the 737 or 320 for rated pilots is 5 days, the full course is a couple of weeks before moving onto the sim stage. I would imagine for ease of planning they would look to place all non-rated pilots on the same course from the beginning to avoid too much lag between the ground school and the sim phase. With that in mind it is logical they would empty the pool of TR'd people prior to giving dates for those undergoing a complete TR course, but the BA training department works in mysterious ways!