I also don't like the online idea.
One other thing you can do is have interested students sign up to bring their own games. (If you have to, have an approved list, or suggested ideas given time constraints, school code, appropriateness, etc.)
In high school for an NHS event we did this and it worked out really well. People brought Trivial Pursuit, Risk, cards (Euchre and hearts were big at the time), etc.
The people who bring the games obviously know the rules and are interested in playing, so that cuts down on having large upfront costs (both in terms of money to buy games for one-time use and in terms of teaching rules to a bunch of people). Students can mill about and play different games, too.
You could then contribute your own games (e.g. Dominion) and only have to teach a few interested students and keep a better eye on your games at the same time.
Of course, these are high schoolers choosing games over sports -- I hope there isn't too much of a concern about them ruining the games or eating the cards, or something.