When passages are learned, you have established a good basis upon which to help build the rest of the memory structure. However, the remaining portion is just as important as the original learning foundation.
To assure proper and adequate review, make out a schedule of learning and review that will tell you, every time you have 'class,' just what will be learned and what portions will be reviewed.
Since forgetting sets in almost immediately after learning, the sooner a passage can be reviewed after the original learning of it, the better. Therefore, what has been learned should be reviewed once or more times each day for many days after originally learning it. After all, why memorize if you are just going to forget it in a month?
In planning your memory schedule, divide the class time into two or three sections, depending on the ability of the children. If you feel the children are capable, new material can be learned each day. This could be the first part of the class. The second part could be daily reviews - reviewing portions learned over the past several days. The third part could be reviewing material learned earlier in the year or in previous years. This time could also be used for testing.
If you only have time to split into two sections you do not need to learn new passages every day. In the first part of the class, you can alternate learning new material with the review of "older" passages. As the second part, have tour daily reviews.
Whatever plan you use, your overall emphasis should not be only learning new passages, but the solid retention of older ones.