Which is correct?
The event was the Shanghai Expo opening ceremony, where the host said it in English asking all to stand up in response to kind of anthem playing thing.
Please, all rise for the national anthem of ........ etc.
But as a short, 3-word phrase, I think "All rise, please" simply sounds neater.
I think "Would you please all rise for..." or "And now let us all rise for..." sounds more familiar, but I agree that with the pause, it doesn't sound abnormal.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.