You are right. You can just practice on specific subject. My son knows the basic but I still started him from Pre-K to give him solid foundation. 1.5 hrs for Pre-K. Kindergarten is almost 5 hours. This is our second day. He loves it!
You can either decrease one grade level up or down in any subject. I liked it.
I did but my son doesn't like it. Two days ago I started him on www.ixl.com for math only and he's hooked. He already spent 4 hours in two days.
I've seen this site before. Can you move between grade levels?