・Has my admission been approved yet?
・Has my acceptance been confirmed?
・I was wondering if my admission has already been granted.
「入学許可は既に下りていますか?」は英語で **“Has my admission been approved yet?”** や **“Has my acceptance been confirmed?”** と言います。
ここでの「入学許可が下りる」は英語では “be approved” や “be granted” などの受け身で表現するのが自然です。