The statement: 'at this school, 'x'% of DCs got 'y' passes at 'x' level', you have absolutely got to look at the intake, otherwise the statistic is meaningless.
If a school has a rigorous entrance exam, one should bleddy well expect them to get a 100% pass rate otherwise one could ask serious questions about their ability to select; if the local comp is getting a 50% pass of all-comers in a 'challenged' area, it's very possibly doing better by its DCs that the 1st school!
Also bear in mind, if a school gets 85% of DCs 'through', where is the guarantee your own DC will be in the 85, not the 15??