Well, everyone above me has made the very valid point that teachers get fed up, too, and it's possible your son has pushed that teacher's buttons one too many times. If the teacher made that comment, then there's probably some history of your son not doing his homework. A teacher wouldn't make that comment for no reason...I think that first, you need to get to the bottom of that situation. How many times has your son not completed his homework for that teacher? (you may have to work to get an honest answer on that one)
But, if you do wish to email the teacher, here are some tips:
1) do NOT be confrontational, if you want your email to be taken seriously.
2) remember that, by emailing the teacher, you are leaving a WRITTEN RECORD of your complaint. (my guess is that this is probably why the district wants you to contact your child's teacher via email, not phone) This can be good and bad. Getting something "on the record" can be important in case there are future incidents. But it's just like what the police tell people they arrest -- "anything you say can and will be used against you." So be very careful on how you say things.
3) Your best approach is not "I can't believe you'd say this to my kid."
4) Find out your school's policy on make-up assignments. Was this an "excused" absence? If so, then he's probably allowed to make up any assignments he missed in a reasonable time period (usually a day). If it was "unexcused" (he was skipping, or not out of school for health reasons or pre-approved educational purposes), then the teacher MAY not have to give make-up assignments. That varies from school to school...and possibly even class to class. You'll need to sort that out first before you accuse the teacher of anything.
5) Your primary concern in this email should be getting your son the assignment he missed. You WILL NOT get anywhere if you try to pick a fight with a teacher over something that she allegedly said (remembering, of course, that you didn't hear the comment first hand...so it is the word of a child against the word of someone who has at least one college degree and a teaching certificate)
6) Keep in mind that your son's teacher is going to appreciate some assurance from you that you are working with your son to make sure he completes ALL his homework in the future.
7) I would say something like this:
"Dear ___________,
My son, _____________, was absent from school on Monday, March 16. He was absent due to __________, and my written excuse for his absence was presented to and accepted by the school office on Tuesday, March 17. My son claims to have been unable to get the assignment he missed from your class. I would like to know how to get this assignment for him, so he can get caught up on the work he missed while he was [ill?]. Please advise, either by return email or by phone. My number is _________.
I know that my son's record of turning in homework assignments has been poor in the past, but I am working with him to fix this situation. [list several things you are doing at home, or will be doing, to get him to do his homework]. In order to help me keep my son on task, I would kindly request that you contact me immediately the next time he fails to turn in an assignment, so that I can take the appropriate disciplinary actions.
Thank you very much for your time and attention to this matter.
Sincerely yours,
_______________"
Hope this helps!