how can admins create users and activate the accounts?

So... knowing user's email, you may log-in as admin, then go to the "Sign Up" menu (your_forums_url/index.php?action=registernew) and register user with his email via regular registration process.

What's so specific between admin and user here?

i want to create a user and issue a password and activate the account

as admin can i create a user, create a password for the user, and then email the details to someone - this is exactly what you need. Or you may set a special option which sends password on user's email upon registration.

how do i do that?

Osiris, you can save Admin a lot of work by just letting users register themselves, and to make sure they are using a valid email address, simply use the 'closed registration' option in setup_options. For more details, please refer to the section on "//$closeRegister=1" in the Manual.

Please note: the link above points to the documentation for the latest version of miniBB, if you are using an older version, say version 1.x, refer to the Manual included in the software package instead.

All the best -
mb

let me explain what i want to do, i want to make the membership of the forum exclusive to students from my school, and i'm looking for methods of achieving that.

So, the only thing you need to know is user's email. Set $closeRegister=1 and register user via usual registration process. Password will be sent to user's email.