Naomi,
The fat content of the milk varies so much that it might be quite a challenge
for your seventh grader to explain.  The best info might be from Paula Meier,
but since I can't find my notes of her lecture, I'll tell you what I can find
in Nutrition during Lactation.

p 118  "Lipids are among the most variable and difficult nutrients to measure
accurately in human milk: among women, the total fat content of 24-hour milk
samples may vary from less than 20 g/liter to more than 50 g/liter."
p. 122  "Using amino acid analysis, Lonnerdahl and co-workers foudn that the
protein content of mature human milk was approximately 8 to 9 g/liter."
p. 124   "In human milk, lactose is present in an aveage concentration of 70
g/liter."

Knowing that fat has 9 calories per gram and that lactose and protein each
have about 4 calories per gram, she should be able to calculate the caloric
content.  I make it roughly <15 to >23 calories per ounce.

I think it's great that now the students are educating their teachers about
breastfeeding!!  You go Girl!!