At this point, there is no student rush for South Pacific. The show is doing so well and selling out that I don't think they'll be doing anything regarding rush in the near future. They did have something called Studenttix in the past, but at this time it's closed. It used to be $20 for tickets for any seat in the Vivian, I used it once for Piazza and sat in the front of the orchestra. But, like I said, currently not available.
I know it's a lot of money, it was my first time paying for a full-priced ticket, but it was worth it. It was a wonderful production. If you plan on doing the cancellation line though, I would get there at least by nine, the box office opens by ten. Its luck of the draw to see how many tickets get sold by cancellation but once you get a ticket, you are free to go... so just bring a book and read

that's what I did for 4 hours haha but I got there at 7.