How to prepare for computer science if the teacher is not good enough to explain

GCSE? Have you raised this with the school? If so and it has not been resolved then complain to Governors.

To help your child, I'd find which specification they are taking and then there are plenty of free online resources. Also maybe consider getting a textbook from the library, look for past papers [on exam board website] and also examiners reports.

We have had this issue with a new teacher for year 11. We downloaded the course details; for the NEA found excellent resources published on other schools websites; the CGP books are good too. I don't see that complaining to the school at this stage would be of benefit to the child and would potentially make his life far more difficult. Perhaps if your child is lower down the school it might be a route you could consider exploring.