I would definitely send a strongly worded email followed by a visit, particularly if you are paying private tuition on top of property taxes. My kids attend public schools and there is basically no teaching by the substitute if it's just a few days. The regular teacher, if she is responsible, generally leaves packets and very detailed instructions on what is to occur during class. In our experience, the only times that a substitute has actually taught has been long-term substitutes. I am assuming that by "a while" your DS means a week or longer so you are definitely justified in being annoyed.