The OP makes it seem like it's an unfair system of making achievers pay for a trip and underachievers not, but the follow up actually show's it's actually slightly misleading in it's lack of context that they are totally different trips organized for the same time with different purposes.
The "under achievers" are being taken on a revision trip to encourage them to work - with the odd incentive activity to keep them motivated. Those who have achieved their targets are being offered a trip, purely for the sake of the trip with very little educational value ...
Since the trips aren't compulsory, I can understand them making the revision trip free to give them no excuse not to attend based on cost since it's designed to help them improve academically. They want to encourage the under achievers, and that doesn't have to mean children who aren't smart or disruptive, just ones who are working below their potential, so they are offering them a couple of days away from school, where they can focus on their studies, but also to do activities which, I will assume, will help them (eg with problem solving/co-ordination/practical application of theory) whilst also giving them an incentive to work towards during their study sessions.
The trip for those who are high achievers, is probably still going to have the same kind of activities to help them apply the things they've learned practically, but it is definitely more of an "enrichment" away day than for educational purposes, like most school trips are.