Most government agencies have an intern/co-op/summer hire program. Congress encourages this, as a matter of fact, to the point that they will often earmark funding specifically for this purpose. The advantage to a student is that they get a head start on a civil service career, because the time spent on the job counts for logevity and retirement benefits. Only agencies that don't require clearances will hire co-ops or summer hires because it takes so long to get someone cleared. A summer hire is ideal for the TSA because screeners don't require clearances.

I may have garbled the definitions, but here is what I recall they are:

Intern: A full-time government employee hired out of college into an entry-level position. These are usually at the GS-7 or (in rare cases) GS-9. There is a probationary period because they don't yet have the on-paper experience to qualify for the job.
Co-op: This is the same thing as a co-op in the private sector. These people work part-time at a government agency while attending a university. There is a maximum number of hours they can work and they must remain enrolled and carry a minimum GPA.
Summer Hire: Again, identical to the private sector -- a person >18 who works full time for a period of a few months during school break. Even though they are full time, they receive limited benefits, but I believe their time fully counts towards logevity and retirement.

In reading the post, I suspect the individual was a summer hire. The benefits to the agency (TSA in this case) are that the person doesn't count against their personnel strength ceiling; and, generally the funds don't come out of the agency's operating budget.