A Team Brownbacker points me to two bloggers who were actually there:

In Brownback’s defense, I was at the event and the photo was taken after most students had left because they had to make their 1:00 classes.

and

Speaking for a mixture of students and professors, Kansas Senator Sam Brownback laid out his Social Security plan. Brownback told the audience he wanted to change the system because “It’s a system that worked great for my grandparents. It’s working pretty well for my parents. It’s working so-so for my generation, the baby boomer generation. It stinks for my kids.” The Senator’s plan would allow people to put five percent of their taxable income into private accounts. William Shipman, an advocate of Social Security reform, joined Brownback.

What: Kansas Senator Sam Brownback Speaks at Social Security Forum

Where: The New Hampshire Institute of Politics at Saint Anselm College.
How many people: 35 people, 20 of which were students
What media was there: AP, WMUR, NECN, Union Leader, St. Anselm Crier, 2 TV cameras, 1 photographer

TB adds:

The photo of Brownback at St. Anselm is highly disingenuous. There were over 50 people at the speech as they came and went over the lunch hour. That shot was taken at the end of the Q&A after the students had left to make it to their 1p classes.

UPDATE: A reader forwards an e-mail exchange he had with a member of the Dept. of Politics at the college. The professor explains: 

There were about 75 people there, including faculty and students. ….
That particular photo was taken towards the end of the event after some 20 students or so had to take off for class. Such is life.