Based on a total number of 91 students whom I have had the pleasure of working with and whom I have entered for exams since 2008:
On average, across grades 1-6:
40% Students obtain a Pass (100-119 marks out of 150 or 67-79%)
34% Students Pass with Merit (120-129 marks out of 150 or 80-87%)
25% Students Pass with Distinction (130-150 marks out of 150 or 88-100%)
1% Students Fail (>100 marks out of 150)*
Broken Down By Grade:
Grade 1: 30% of students Pass; 40% obtain Merits and 30% obtain Distinctions
Grade 2: 57% of students Pass; 29% obtain Merits and 10% obtain Distinctions*
Grade 3: 64% of students Pass; 18% obtain Merits and 18% obtain Distinctions
Grade 4: 50% of students obtain Merits; 50% obtain Distinctions
Grade 5: 33% of students Pass; 33% students obtain Merits and 34% obtain Distinctions
Grade 6: 25% of students Pass; 25% students obtain Merits and 50% obtain Distinctions
*One student failed at Grade 2 (2010) showing an anomoly failure rate of 5% at this particular grade.
Compared to ABRSM Statistics (relating to their most recent data of 2012, to all practical exams taken and in all countries):
Grade 1: 38% of students Pass; 39% obtain Merits; 21% obtain Distinctions; 2% Fail
Grade 2: 47% of students Pass; 35% obtain Merits and 15% obtain Distinctions; 3% Fail
Grade 3: 48% of students Pass; 35% obtain Merits and 14% obtain Distinctions; 3% Fail
Grade 4: 51% of students Pass; 33% obtain Merits; 12% obtain Distinctions; 4% Fail
Grade 5: 49% of students Pass; 31% students obtain Merits; 13% obtain Distinctions; 6% Fail
Grade 6: 46% of students Pass; 32% students obtain Merits; 17% obtain Distinctions; 6% Fail
Source: http://us.abrsm.org/en/press/exam-statistics/graded-music-exams-by-result-practical/