How good (at English) do I have to be before I can move into the top class?
What level of English do I have to reach to be moved up to the top class?
「どれくらい上手くなったら、上のクラスになれますか」＝ How good (at English) do I have to be before I can move into the top class? / What level of English do I have to reach to be moved up to the top class?
「上のクラス」＝ top class/high level class/advanced class
「どれくらい上手くなったら」＝ How good (at English)do I have to be/become.../What English level do I have to reach...?
イギリス 
