Galleri Lafayette is probably the most eyecatching, but I voted for the Eiffel tower girls because it emphasises the social aspect (girls talking) in foreign environment (made obvious by the Eiffel tower) which is always effective in these situations. It also has a nice composition and is very symbolic of the whole study abroad concept.

Does the contest disallow photo editing? If not I would spruce up the contrast and maybe brightness and saturation as as well to make it more effective. Try using Auto Levels if you have Photoshop.