I would suppose (however I am not sure here) that following method calling getCallerPrincipal() will return "student" and "notebook" (or at least student, when calling notebook's principal - in the case below) values and not "anonymous" value. From EJB spec in 12.3.4.1 Run-as:

The Deployer then assigns a security principal defined in the operational environment to be used as the
principal for the run-as identity. The security principal assigned by the Deployer should be a principal
that has been assigned to the security role specified by RunAs annotation ...

Note that I created ApplicationUsers, which maps to the roles of same name (student=student, etc. in application-roles.properties)
This could be IMHO somehow enforced. In JSR-250 2.8
javax.annotation.security.RunAs there is:

The RunAs annotation defines the role of the application during execution in a Java
EE container. It can be specified on a class. This allows developers to execute an
application under a particular role. The role MUST map to the user / group
information in the container’s security realm.

@Named
@Stateless
@RunAs("student")
@SecurityDomain("other")
@PermitAll
public class Student {

    @Resource
    private SessionContext context;

    @EJB
    Notebook notebook;

    public String getNotebookPrincipal(){
        return notebook.getCallerPrincipal();
    }

    public String getStudentPrincipal(){
        return context.getCallerPrincipal().toString();
    }
}

@Stateless
@RunAs("notebook")
@SecurityDomain("other")
@RolesAllowed("student")
public class Notebook {

    @Resource
    private SessionContext context;

    public String getCallerPrincipal(){
        return context.getCallerPrincipal().toString();
    }

}