All I'll say is this...
My first bike was a D675...crashed it many times. Eventually picked up track riding after thinking I was hot stuff in the canyons.
Started doing trackdays...was humbled very quickly even in the beginner group
After doing a handful of trackdays, I did a mini trackday with an NSR50...dragged knee for the first time and actually learned the value of RPM management and preserving momentum.
Right after that, I did a trackday again in C Group and was immediately bumped up to B group. In the B Group I was within the mix.
After doing about 5 more trackdays in B Group, my pace was fast enough to be in the advanced group.
So after about 10 trackdays, I was and now currently ride in the advanced group.
I would say the Grom is great bike to learn on...especially if you are trying to teach someone young.