From my point of view: Their are two reasons for the increase of slow learners: 1) The curriculum. 2) The teachers (and their teaching methods)
In relation to the first (curriculum) - curriculum can be organized using a basic core as foundation for all pupils (including contents and skills) and then widening this core so as to enable pupils to reach higher levels of understanding and performance.
In respect to teaching methods, there is much to be done, basically because teachers teach "contents and skill" but they should teach pupils and learn how pupils learn. In this situation the learners can be provided with situations apropriate to their learning types, which do not create "slow learners". We havve a very successful experience in both issues and are willing to share them with anybody interested. (This is not a commercial offer it is a pedagogical one)
The important thing is to combine 1) with 2) providing an interesting, organized and supportive learning environment which enables pupils to get in contact with mathematics and grow in the field.