Need help form inherits

Hello everybody,

I would like to use inherits between 2 tables :

user(login,password)

and

studiant(firstname, lastname, email)

these tables are created like this :

create table user (
login varchar(15) not null primary key,
password varchar(32));

create table studiant (
firstname varchar(35),
lastname varchar(35),
email varchar(255)
) inherits user;

The problem is that a studiant can become a user of the site to consult
its results. Members of user table are actually secretaries, who
administer exams ...

so, if I create un user :

insert into user values ('por',md5('tototo'));

and I want know to say that the user 'por' becomes a studiant. How can I
do this ?
In the other way, to say that a studiant is also a user, I just have to do :

insert into studiant
('kml',md5('tititi'),'Michel','Michelle','michelle21(at)dynu(dot)com');

And all is ok.

Thanks for your help.

Patrice