Read Desirée Rumbaugh's response
There are definitely challenges presented when we attempt to bring new students into our existing classes. And yet, at the same time, we all want our classes to grow and to accommodate the ever-changing schedules of people's real lives. New ones will come and some others will always leave.
The easiest and perhaps most ideal solution would be to add a beginners' class in addition to the now mixed-level class, even if the beginners' class was taught by someone else with a compatible teaching style.
If that is not possible, then there is a way in which you can graciously welcome the new students while honoring the continuing people. It is actually to their benefit that these continuing students welcome and assist the "newbies," as it gives them a chance to practice generosity. Their own practices will deepen in many ways as they learn to assist others and set an example for them. Simple partner work or just being steadfast in their own hearts, minds, and bodies can actually bring the new people along faster. Much of this will depend on your attitude and how skillful you can become at giving instructions in a clear and simple way. If you have the attitude that growth is the whole point of yoga classes and that becoming more open-hearted by helping others is the point of the yoga, then the others will catch on to that mind-set too.
It would be helpful to discuss these changes briefly at the beginning of the first class of the next session, in order to set the tone. My guess is that you will find that the group comes together harmoniously and easily if you set this intention as their guide. Also remember that, in a class that meets only once a week, it is doubtful that the students who have attended only six classes are really doing anything at this point that would be dangerous for the new ones to try.
Many blessings to you as you grow a stronger and more vibrant yoga community with the coming of each new session.