At schools, it really depends on what the teachers knows. When I was at school, I learned Delphi and PHP and JavaScript. Other people told me that they learned C# or Java or Visual Basic.
But if you want to know, why Java is such an important language, the answer is quite straight. Because Java is very easy to use (e.g. no pointers, no type-hell,...) and it was made for enterprise usage. As a programmer, chances are high you will be employed by an enterprise using Java.
Aside from the enterprise thing, Java is widely used by a lot of people and readily available for nearly every platform you might ever encounter. So by teaching Java, teachers (and later professors at University) give you a good start for anything you might want to do.