You won't catch up with your classmates u____ you do it day and night.

很明显，unless，除非的意思

You won't catch up with your classmates unless you do it day and night.
你不会赶上你的同学的，除非你日夜地工作。

You won't catch up with your classmates unless____ you do it day and night.

unless或者untill
除非你夜以继日地做,否则你就赶不上你的同学了

unless
除非你夜以继日地做,否则你就赶不上你的同学了.

unless