Well, she could split the class and have a beginners' class and an advanced class; but that's not always possible (budget, staff availability, etc).
Otherwise, she needs to prepare two lessons -- one for the advanced students, and one for the less advanced. The lessons need to be structured so that one group can be given work to do on their own while she gives instruction to the other group -- and then vice versa.
Also, she can devise activities where the stronger students support and help the others in some way.
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