Java's not the easiest to pick up in high school unless you really make a big after-school effort. Something like Lua is probably better.
But I have long been of the opinion that both languages are kind of an ill fit for the application. Python is making some headway and I think it might be a better fit; It has some advantages on legibility, it has an optional static typing system, and it does support coroutines.
A bit of an aside maybe, but the guy that made Quasar is behind project loom to add light weight threads to JVM which will become available in JVM 21 https://openjdk.org/jeps/444
The right intro language is an interesting study in itself and the intuitiveness of coroutines is an good data point. Go seems like a decent first one. It jas dark corners, but at least they are in the corner. I'd love to argue for rust as a first language, and maybe it isn't a bad one. But I'm not sure where I'd start the argument. C was my second language, and I'm not sure it would have made sense as qyuckly before commodore basic.
Java has its domains, but small, algorithm driven code worked on by a small, inexperienced group isn't one.
When we were still using Java, I had the students just make everything static. We don’t need more than one instance of our IntakeSubsystem. We only have one intake. But the OOP boilerplate persists.