Wow - where to start....
First off, you say "don't fall into the trap of using Java as your core language ... only know how to program in a language that almost nobody in the industry actually uses"
That's just completely wrong. Java is THE most used language in industry. Here's a completely unscientific method of finding out, but I did a search on monster.com for "x developer", where "x" was java, c++, c#, c, and objective-c. Here's the results:
Java: 4035 jobs
C++: 2022 jobs
C#: 2966 jobs
C: 1457 jobs
Objective-C: 31 jobs
Given that C# is pretty indistinguishable from Java as far as programming language (not libraries) goes, by learning Java you get not just the first most popular language (Java), but the second as well (C#).
Then: "you might consider teaching them Objective-C" - so yea, you get less than 1% of the job market of java.
That said, I think "job market" is a poor argument to make here. No one (I hope) is going to go out and get a software development job out of high school. For developers, you want them to start thinking like a programmer (Java is great for that, by the way) so they can get into more advanced courses in college easier.