(a fourth grade example)
(another fourth grade example)
(a second grade example)
(another second grade example)
image via how to draw and paint
On a piece of scratch paper have the kids draw an oval, then divide it in half horizontally and vertically. On the board you do the same, and then add in the eyes. Divide the space between the eyes and the chin in half horizontally, and add a nose (see drawing above). With your child model, point out then how the tops of the ears line up with the eyes, and then how the bottoms of the ears usually line up to the bottom of the nose - sketching them in like the drawing above.
Then, divide the space between the nose and chin in half horizontally, and add in the the mouth. Use your child model to show how the ends of the mouth line up with the middle of the eyes.
Once your practice drawing is made, give the kids a large piece of drawing paper and have them pencil in a large oval, and a light horizontal and vertical line to help them know where to put the facial features.
Now invite the teacher to the front of the class and have her be your model. Point out details like her hair color, if she is wearing earrings, her hair length, etc. Then have the kids go for it - first drawing with pencil and then adding color with pastel.
(Optional idea: while the kids are drawing, sneak around the class and ask the kids what they think is most beautiful about their teacher, and write it down. Then at the end of the lesson, take all of the pictures and instead of putting them in the kids' portfolios, make a book of them to give to the teacher, writing the quote from the child under their drawing. The collection of drawings makes a great class present in honor of the teacher's birthday, or teacher appreciation week.)