Which is in any case a better test of the core of coding, which is about figuring out algorithms and data structures, not writing glue code to connect various APIs together.
The challenge is figuring out how to provide a novice (high school students) with a skill appropriate challenge to assess them on that is also not trivially solved by the use of an LLM, and you likely can't. Or you go the IT route and restrict what's available on and from the computers (see my comment about how ACM and IEEE contests used to do things, they may still run that way but I'm uninvolved so don't know).