In my opinion, you should go ahead and create a database. Making example code based on a text-file is all good and well, but pretty much all of that code will be useless once you get a database to store the data in. You will most likely rely on a strongly typed dataset and use SQL to retrieve:
* Logins for a specified person (possibly over a given period in time).
* Logins for a specified period.
* Missing logins for a specified person (possibly over a given period).
* etc.
eliminating the need to store text-files and/or declare classes or variables to maintain the data.

I implemented a similar system for a youth-centre a couple of years ago. The data basically consisted of a table of persons, each uniquely defined by an id, and a table of logins using inner join on the person-table on the id and keeping a login-datetime (in your case also a logout-datetime or possibly login-duration). Each young person had a magnetic card, and when they scanned it in a given club, the information was sent to a central server storing the information for all 21 clubs.
The system allowed for a wide range of statistics to be drawn, cards being banned etc., but basically it is similar in nature to your project.

Start out with a very simple database consisting of just 2 tables with as few columns as possible - for example:

Person_Table:
* Id (integer, primary key)
* Name (string)

Attendance_Table:
(* Possible autonumber for primary key or no key at all - depending on whether you plan on deleting/editing rows or permanently keep them as is).
* Person_Id (integer - refers to the person-table)
* Login (DateTime - the date and time when the student arrives).
* Additional columns can be added as needed, but no more columns are required to make a working example.

Once you have the database created with the 2 tables, you will find that using databases is pretty well-supported and not that difficult in the .Net framework. The most difficult task (IMO) will be creating SQL-sentences to extract the required information - ie. attendance in professor X's classes in May, number of days Bryan was absent in any class in 2012 etc. etc.

Regards Tom