Self-Paced learning?

In my 4th Grade clroom I have a self paced math group. These children work at their own speed and are usually 1 chapter ahead of the rest of the cl. I formed the group after noticing that these kids were becoming frustrated with having to wait for the rest of the kids. Since forming the group, they are much happier. Initially, my other students cried "no fair" because these students usually get up and leave when we start math. I pointed out to them that fair doesn't mean everyone is treated the same, fair means that everyone gets what he/she needs to succeed and this is what they need. The cl is OK with it now. It's a fluid group. Kids can join, then opt out when they get to a concept they have difficulty with.