Selling to the parents is definitely one method. Selling it to the Teachers/Counselors I would think is another strong approach. The students teacher will most likely be the first person to notice when a student is in need of a service such as yours. Presenting your ideas to the faculty/staff of local schools may be a way to get your foot in the door.
Also a side question that I have is will this be a "snail" mail assignment? In todays digital age I would not limit myself to paper mailings. Lots of unnecessary costs involved with creating/printing/mailing assignments if the student/parent is capable of receiving it via email and printing it at home.