Concurrency Issue On A Single Database User For An Online Read/write Application

Hi to all DBAs,

I would like to ask if there will be a write and concurrency issue if i would create an online application with just one user connecting to the database, just like most open source php/mysql that can be downloaded

i.e
Setup 1. I will grant a single user that will connect to a database and will be set to a config.inc.php file and then create a table users (userid, username, password) and this table will be used for the application authentication and access control.

Setup 2. or is it much better to grant users that will connect to the database and have the table users for access control of the application?

to further illustrate my query:

Setup 1 would be:
###################Table: Users
user a@ipadd --> db1 --> userid mary --> myOnlineApp
user a@ipadd --> db1 --> userid john --> myOnlineApp
user a@ipadd --> db1 --> userid paul --> myOnlineApp


and Setup 2 would be:
#####################Table: Users
user mary@ipadd --> db1 --> userid mary --> myOnlineApp
user john@ipadd --> db1 --> userid john --> myOnlineApp
user paul@ipadd --> db1 --> userid paul --> myOnlineApp

will Setup 1 be enough if I will use SQL Server as database?

or Setup 2 is better user/database architecture?

This application will be online enrollment for a school with 16,000 students

very much appreciated for the feedbacks and suggestions

^_^x