I´ve used studentuniverse.com in the past to book flights once or twice and it has been reliable for me with no incidents.

I found that occasionally they have deals that you cannot find anywhere else, specially for intercontinental trips (including one-way trips). Not sure how they do it, I assume they have special partnerships with certain airlines. The site is aimed for students, but they have the option to book as a regular adult at the same fares.

I would be glad to hear experiences with them from other people.