Be very very careful!
I am speaking with over 30 years of classroom experience, and would like to encourage you, however:
1. You must meet with the school administration...principal etc. also you will probably have to have district legal approval.
2. You do not directly approach parents! The school district will have a distinct policy on this and if your project is given approval, they will provide the legal documents to be given for parental approval.
3. A third very strong caution!
You are dealing with special ed students!!!.... and another set of criteria on privacy comes into being. Federal mandates and protections are in force...you wife should also approach her immediate supervisor etc.
4. What is the purpose of the photographs?
Classroom use for student motivation? or are you planning to hang a show of special ed students!
Remember dealing with students in a school settiing is not the public.
5. In most school districts, unauthorized personel are not allowed in the classrooms until checked through administration, that now even includes parents! After 911 even more precautions are being added!
Please express your intent and the use of the photos to the administration and you will get your answers....If you were to photograph students, hang a show or publish without proper disclosure and permits, your wife's position would be in jeopardy and you and the school district would be subject to suit,
Now that being said, I would encourage you to procede....and let us know how your project evolves...A good journalistic approach to a special ed classroom is certainly a worth while project.
I recently was involved in the production of a very positive promotional documentary film of the drama dept of a school here in Las Vegas, using both film and analog "stills" 35 mm and 120 ....even though it was a project for the department and the school administration I had to go through all of the above....there actually were parents who opted out of having their children shown in the film and would not give release!
Good luck
Dave in Vegas