As was said, usually the higher number equates with fancier or more curl.
If I were going to this school, or even if I wasn't going to the school but intended to build a Kentucky style rifle, I would buy the highest grade
I could afford . If it wasn't as fancy as the other students or the instructors wood, that's the way life is. At least I would have the best I could afford.
I have learned over the years that it is unwise to buy less than you can afford. This is not because the plainer wood is of a lesser quality. It is because after you put all of your blood, sweat and tears into the gun you owe yourself the best you can afford.
Several guns I've built were intentionally plainer grained than I could afford. I usually had some great reason, like, "This is a boys rifle. His old man wouldn't have paid high prices for fancy wood for a kids gun." Or, "Plains Rifles usually didn't have highly figured wood, so I'll by the low priced stuff and save money and be PC at the same time"
In every case, I kicked myself in the butt for being so "PC" and wished I would have used wood with a fancier grain.