Blog post alert: The British Library is digitizing manuscripts pertinent to the lives of women in the Middle Ages. Hildegard-go! reports on ten manuscripts and contains this illustration of girls learning to read in a classroom. A link to the full manuscript, a Dutch prayer book, lets you view it and the page opposite (f. 28r), which includes an alphabet.
An article at Medievalists.net, Learning to Read and Write: Women's Education in the Middle Ages, includes a discussion of schools for girls illustrated by the Harley MS 3828 image given above. Another easily accessible article posted by the University of Notre Dame answers the question Could Medieval Women Read? And for those of you with access to JSTOR, a 1993 article, "The Wise Mother": The Image of St. Anne Teaching the Virgin Mary examines the topic of mothers teaching daughters to read in the later Middle Ages. You can see an example of the motif at the Getty.
What I haven't found is another image of a class of several girls being taught to read together. Help, anyone?