If you don't work hard, you won't catch up with your classmates.

原句主句和从句都是否定句，那么都把他们改为肯定句，和原句意思是一样的。
原句
If you don't work hard, you won't catch up with your classmates.
改后
Work hard,and you will catch up with your classmates.
当然也可以
If you work hard, you will catch up with your classmates.

work hard, and you will.......

Work hard ,and you will catch up with your classmates