The difficulty with teaching the required curriculum in Gaelic is that you end up with a situation where probably 90% of the texts are in English. This results in any number of learning difficulties for the students. For example, imagine teaching (or learning) chemistry when the lecture is in Gaelic and the text in English. Foreign languages magnify the problem; lectures in Gaelic, the text in English, and the translations in, say, German.

While I'm all in favour of the children having the ability to speak, read, and write, in Gaelic, I firmly believe that a "Gaelic only" school does in some ways reduce the quality of learning. Certainly in Ireland (where my son went to school) extra leaving certificate points (a 10% head start" bonus) had to be assigned to those students who completed their leaving cert. (similar to the British A Levels) in order to bring them to parity with those children who were taught in English, when applying to university.