How to create an email group easily in Mac OS X?
Typically, if you had many people from your Contacts you would simply create a group by following:


Open Contacts.app and select "File" -> "New Group"
Enter the name of the group you are creating.

This is what you will type into mail, and will effectively be the name of your "distribution list".

Go back to "All Contacts" and drag the contacts in that you would like to be in your group.
Now, when you want to sent to that group, just type in the group's name and Mail.app will automatically populate the sending information.


However, for your situation, that won't work very well as you need to create a distribution list for many people who aren't in your Address book (per your edit). With a tip borrowed from iOS, and a hack, you should be set.


Your list of email addresses must be comma-delimited. If it is separated by line breaks, then this likely won't work. So first we need them comma delimited. 
Now we will create a contact, and name it "2013 4Q Students" or something.
Next, we will put our comma-delimited list of email addresses into one email field. 

I assume there is an arbitrary limit on how many email addresses you can put into one field, but I don't know from experience what that might be.

Type the new "contact" name into a Mail.app compose "To" field. It will come up with the standard blue bubble and all those email addresses will be packed inside.
Now we proceed with a small hack: click the down-arrow next to the contact name in the blue bubble. 
Click "Edit Contact".
Remove the contact name ("2013 4Q Students") from the beginning and the "&lt;".
Type Command + right arrow to reach end of email address line, and remove the ">".


Just a little cumbersome, but this should work for you.