Quick engaging card games that can be learned quickly by a new group
Fluxx springs immediately to mind: almost zero teaching time ("The rules are on the table!  Draw a card then play a card!"), can easily be over inside of ten minutes, inconsequential enough that no one will really mind if the bell goes and you have to quit before anyone has won the game.

Lunch Money could probably work inside the given timeframe, but a disturbing game about children beating the crap out of each other in the playground might get you some strange looks, or even disciplinary action, in an academic environment :D

I thought of a couple more good options, so I'm editing them in:

Parade has really taken off in our group lately - it's simple but brilliant.  There are 6 suits of (Alice in Wonderland-themed) cards, valued from 0 to 11.  Beginning with a "parade" of 6 random cards, each player adds a card to the end of the parade.  You must then add to your collection all cards that are equal-or-higher-valued or same-suited as the card you just played - except for the n cards immediately in front of your card, where n is your card's value.  (i.e. if you play the 10, you pick up nothing unless the parade is now 12 cards long.)  At the end of the game, you score negative face value of card, UNLESS you have the most cards in a suit, in which case they're only -1 each.  Trust me, it's really good.

Coloretto is a simple card game which happens to be the engine behind popular boardgames like Zooloretto and Aquaretto.  Players either draw a card and add it to a pile or take a pile.  Most of the cards have a colour: at the end of the game, you score positive points for cards in 3 of those colours, and negative points for cards in other colours.  Pick up piles too early and you may end up short of cards; wait too long and the other players will screw you by leaving you with a pile of cards of multiple different unwanted colours.  Simple but a lot of fun!