Hi, I'm a sponsored prep candidate and I can tell you that right now, those who haven't got their appointment at the prep school yet are mainly those who haven't met the contracted GPA. They are waiting until finals (around May 3rd ) to hear back from the academy whether they have meet the contract and will be appointed or fail to meet the contract and receive the letter of declination. I would assume that they should have notify the self-prep candidates by now if you're appointed or not.
I contacted the admissions earlier this month and they said those who haven't heard back should be hearing back NO LATER THAN THE BEGINNING OF MAY. Around this time last year, I have received my sponsored spot at a prep school from the USMMA. Hang on tight and don't give up yet until the fat lady say so.
Good luck!