As a recent student, I can say that I never had writing help on any of my papers. In fact, I helped other students with theirs, in an editorial and idea sense. I often had my Dad listen to and suggest edits, but I always wrote every word myself.
In the sense of help -- which is to say, coming up with ideas, outlining, editing -- I have no problem with parental assistance. It's a good way to reinforce grammar rules, sentence structure, punctuation and spelling... all the things that children pay little attention to in school because it's so darn boring. I should know; I got most of my writing knowledge from reading, not schoolwork.
However, in any case where a teacher can prove beyond a doubt that the student did not write the paper him/herself -- for example, if the teacher quizzes the student about the paper's content and the student cannot answer specifics -- the paper should be rejected and the parent called in for a conference. Teaching children that someone else will do their work for them is a bad, bad lesson, and it leads to helpess adults with no sense of responsibility.