Well, dd was 8 when we tried it, and because we kinda unschool, she had definitely not had a lot of exposure to formal writing. I didn't want her to be intimidated, and I was pretty clueless about the extent of her verbal abilities, so I signed her up for the sentence course. It wasn't a lot of work. It was very simple. Fine for what it was, but it didn't really engage or challenge. Then again, maybe that is because I chose the wrong course. But I did find it very basic and formulaic. It is not designed for gifted kids. Teacher feedback was "Great job!!!" how much can you really say about a sentence, I know. But still
At one point, dd got really sick with pneumonia for 10 days and we fell somewhat behind. I communicated with the teacher about her illness. When she recovered we inadvertently submitted two assignments in one week. You were not "allowed" to do that, but I think I lost track in my effort to have her catch up. The teacher wrote me a sharply worded e-mail reprimanding me. It felt unnecessarily censorious to me. A kind reminder would have worked fine. That left a sour taste in my mouth.
