Ever been to Paris or France and seen what's on offer over there? Once they visit windy, windswept Giant's Causeway and experience the disappointment most people feel when they arrive there they'll never be back.
Afraid it's completely over-hyped. Harsh but true.
Poor marketing (again) by whoever does it

Probably just good old fashioned teaching methods rather than the new fads like using 'number lines' to add up (rather than simple add-up and carry-over like we used to do...) or encouraging competitive sport rather than the 'everyon'es a winner and nobody is a loser' approach..I'm sure good old-fashioned teaching methods are also alive and well in the non-catholic schools that are doing well...

As a working class boy who benefitted greatly from the grammar school system and now lives in England, I feel the need to remind NI people just how good the education system is 'over there' rather than 'over here' because of the grammar school / selection system. A previous comment that a move to the comprehensive system is moving everyone to the lowest common denominator is absolutely spot on - I've seen it in action over here and it's frightening to think of what could happen if applied in NI. It doesn't matter that the schools doing well now are Catholic or otherwise, I'm sure the other grammars aren't far behind and will take their turn at the top of the pile in due course. Please, people of NI, do not let your education system change, it's a jewel and standout feature of NI life. If you change it you will be so, so sorry...