If there will be a specific showtime, then you need the Silver package (and to purchase a performance royalty for that specific performance) or higher. However, if the conference would like to make the performance available throughout the conference period, then you'd want at least the Gold package, which allows for on-demand viewing (along with one performance royalty).
However, let's say you want to produce the play at a specific day/time at your school (for example, Thursday at 8 PM as a preview performance) and then make a recording of that same performance available for on-demand viewing at a conference on Friday and Saturday. In that case, purchase the Gold package along with a single performance royalty. Your Thursday 8 PM show would be the licensed performance, and then the conference would be considered on-demand streaming (you get up to two weeks of on-demand with the Gold package).
If you have already used up your on-demand time, then please purchase one royalty for each day your play would stream at the conference. So if you conference is Friday and Saturday, that would be two performance royalties.