It would be really useful to be able to schedule individual pages to publish at a specified time.
Having said that, I can suggest a solution for your specific issue (I think). I use this when I want a quiz to include downloading a file during the quiz.
First, you have to have student access to Pages disabled. That way, students can only access pages you add to Modules or link to directly. Then, you add a link to the page within the Assignment. That way, the page will only become visible to students after the Assignment is open & they enter it. One additional note: the page needs to be published, but it doesn't need to be in a Module.
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