"....having already passed Grade 8 and played a Mozart Concerto with...."
For the benefit of our collegues across the pond, this is not a school year - it is an ABRSM exam. You start at Grade 1 then all the way up to Grade 8, and then on to Advanced Certificate.
To give a quick idea of the standard, Grade 8 pieces include the Poulenc Elegie and the Vinter Hunters Moon
Whenever English people start mentioning 'grades' with reference to instruments, we will almost always be referring to these ABRSM exams. When referring to school age it'd be something like "..in the third year at school.." or something obvious like that :)