I did my PhD in crystal growth of sugars and organic acids. The problem you have is that yes you can make a highly concentrated sugar solution, but if its too concentrated the solution will become very viscous, so even if you add something to initiate crystallization like a string, the molecules can't move quickly enough to form crystals at the size you are envisioning. So for the most rapid crystallization you need to have the solution be concentrated enough without it being too viscous. Heating reduces the viscosity, so its possible in your class the crystals formed as it was cooling.
What may have happened in that experiment you remember is less that the crystal formed instantly, but that it formed as the class was progressing. The tap on the glass would have done little to form it. And even then its lucky it did at all. Sugars are quite difficult to crystallize predictably (wide metastable band widths, probability driven nucleation within the metastable zone, which is why you add a string to get it going. Look those terms up if you're interested).
Tapping on the glass of a supersaturated solution sometimes initiates a crash, but rapid precipitation results in small crystals, not large. The instructions online tell you it takes days, because you have to be very careful when growing a large crystal, that you don't crash out the concentrated solution and form small crystals around or on the surface of the large crystal.
You can test the tapping experiment by putting a bottle of water in the freezer for a while, then carefully taking it out and tapping on the side. If the water is supercooled sufficiently, it will rapidly freeze. That might be an interesting one to try with your grandson.