I use TG all the time for my keyboard pupils and have been very happy with them. In particular I have been impressed by the wide range of supporting tests now on offer. However, a couple of my pupils have reported rather unexpected experiences with the improvisation tests - eg one was doing a melodic improvisation and appears to have been given a piece of paper with the starter notes on, whereas I gathered from the syllabus that these were played to the candidate who then played them back by by ear before preparing the improvisation. Another appears to have been given a different test to the one I prepared for him and I'm not sure yet how that happened.
It's always difficult to know what has gone on in an exam situation when you are reliant on the accounts of children, and I think I will probably contact the board to try and get a clearer idea of what is required, but I am interested to know how other teachers have got on with the improvisation tests.
