Why a new user cannot be directed to the authorized page right away?

 Hello, friends,
We have an asp.net 3.5 (c#.net) web application. We want to set security for certain directories in web.config like the follows:
 <location path="Student">
      <system.web>
            <authorization>
                  <allow roles="Student"/>
                  <deny users="*"/>
            </authorization>
       </system.web>
</location>
This works fine when a user logs in.
However, if a new user just signs up in NewUser.aspx page, the app can not directed us ...