Java isn't impossible for students to learn. I suggest, however, that it is not the best choice; because when it comes to learning how to program you want a language which is maximally expressive with a minimal amount of work. It artificially restricts the problem space you can tackle by making simple things more complicated than they need to be.

When you're writing an algorithm to count words in a file, would you prefer

  string = File.read 'path'

or

  StringBuffer fileData = new StringBuffer(1000);
  BufferedReader reader = new BufferedReader(
          new FileReader("path"));
  char[] buf = new char[1024];
  int numRead=0;
  while((numRead=reader.read(buf)) != -1){
      String readData = String.valueOf(buf, 0, numRead);
      fileData.append(readData);
      buf = new char[1024];
  }
  reader.close();
  string = fileData.toString();

How about introducing anonymous functions? How much time will it take you to explain the intricacies of creating an anonymous subclass whose only purpose for existence is to encapsulate a single instance with a single method, inside which all variables from the containing scope must be marked as final? Or would you prefer something like Smalltalk?

  [ :x | x * x ] value: 2

Sure, students can wrap their heads around both. But the important thing is the concept, not the expression. After all, if they go on to program in the real world, they'll be adapting their knowledge to different languages.