I regret declining a PhD offer and accepting another PhD offer. What should I do?
I feel very regretful now.  ... Should I ask school A to accept me one more time or reapply?


If you prefer School A to School B, then I doubt anyone will begrudge you a new application to School A.  Since you have taken a deferred entry at School B, the inconvenience to them of having you leave is less than if you had started your program.  If you decide to re-apply to School A then you will need to decide whether you want to quit School B first, or hold on to it as a fall-back.  In the latter case, you will also need to decide if you will disclose your new application to School B.

I can't speak for others, but if I were your proposed supervisor, and you had not started your candidature yet, I would have no problem with you coming to see me and telling me that you want to apply to another school.  Even if you wanted to do this, but keep your existing deferment as a fall-back, that would be fine with me.  (We deal with ranked preferences all the time in student admissions, job applicants, etc.)  I would appreciate you being up-front about this, and talking to me about it.  It is an inconvenience to lose a student that has been admitted, but it is not a big inconvenience, and it is far preferable to having a student trying to grind through an arduous PhD program in a school where they are not happy even before they start!

Now, having said this, obviously that puts you on the radar as someone who might not enjoy it here if you end up in the fall-back position of attending School B.  If worst comes to worst, and you are rejected by School A, after disclosing your application to School B, you might have to have a talk with them about whether you still want to be there.  Nevertheless, you can cross that bridge if and when you come to it.


  Applying to other schools might violate the deferring rules of School B.


It "might".  But here's a pro tip --- PhD students are expected to be able to find documents and read them, so exercise those skills to go ahead and read the rules and see if this does violate the rules.  (I doubt it would; there would almost certainly be legal issues relating to restraint-of-trade if a university had rules preventing you from applying elsewhere.)