Sunday school is often good for kids, but if you look at most of the material put out by places like Lifeway then you quickly find yourself questioning why you are wasting your time on a Sunday morning. Yes, it can be a good form of a Bible study, but it most often stops there. It turns into an additional hour on Sunday mornings either before or after hearing the sermon. But what people need is not another hour of teaching because that usually results in overload and by Wednesday people do not remember much of either one.
For most churches in the modern era the answer to Sunday school has been small groups. Small groups are much more laid back as they typically meet in homes, involve snacks, and people are more casual. I do think that small groups are a better form then Sunday school, but I have this sense that for many churches small groups have just turned into Sunday school at someone's house or another Bible study. That is the reason that I see neither as the answer. An interesting fact is that in the last two years I have been both a Sunday school teacher and small group leader, but with missional intentions,;).
Instead I am a proponent of being/living in a missional community where every disciple of Christ sees themselves as a missionary wherever it is that God has placed them. Typically a Sunday school or even small group turns into something that is on your list that you do once a week. Being/living in a missional community is your life, not something that you add to it.
Currently I still lead a "small group", but we have focused a lot on seeing ourselves as missionaries in RDU and transitioning into a missional community. It must be said that this has not happened overnight and in many ways is still happening. So if you are in a group, whether it be Sunday school or a small group, do not get discouraged because it does not happen overnight, but eventually each group member will see themselves as a missionary and collectively you will start living your lives on mission with God's mission.
Neil Cole presents The DNA of a missional community as:
Resource of the Day: http://www.vergenetwork.org/