No your statement shows you have no idea on selection or how to coach a team. Given this was the first CAS game of the season you are suggesting that coaches and selectors have already made up their mind on who is in the team and both trial form and improvement are irrelevant. This is the pet hate by all school boy rugby players that coaches have already made up their mind on who is in t he 1st team even before a ball is kicked. I certainly hope that schools with depth and tradition give each rugby player an even chance on selection
Never in my many years of observing schoolboy rugby have I seen a winning side dramatically change its lineup over the course of the actual season except in the event of injury. The formula of selection you have suggested doesn't make sense to a side that wants to win. Schools with depth - ie Knox and Barker, generally bring their 2nds players off the bench and they usually train their 1sts and 2nds together in the pre season as I'm sure many schools do.
Most of the experimenting etc that you think is necessary in order to be 'fair' to all students happens in the pre season. Further, as I mentioned before, if any player is playing resoundingly well in 2nd grade it is likely that he will come off the bench in the 1sts at some stage - if he plays well in the 1sts then I'm sure that the coaches would consider keeping him in there, but this situation is rare. Other than this what's your point? Because mine was quite simple - 2 starting players, that had trained and played in the 1sts for most of the preseason and the weeks leading up to the 1st round, weren't allowed to play. This would have a significant impact on any side, whether it be in the set plays, in general play defensively or offensively, or for general team morale.