Read section 4.3 - this says that session is the default scope.
The source code also shows the logic of "if request is specified, set to request else set to session". Finally, the default value in the ActionConfig class is:
protected
String scope = "session";
Anyway, it's hard to say from the surface what could be causing the issues. I do admire your patience for how many times you've probably restarted your server on each tweak of the struts-config file.
There has got to be something deeper causing the problem.
It seems weird that you are using a map-backed form to display information about a single student.