The team who will get the highest score wins.

[diamondcutter]

Our school is going to hold a contest in which every class sends a team of 20 students to sing a song together on the stage. After singing, each team gets a score. The team who will get the highest score wins.

(By me)

I’d like to know if the last sentence should be rewritten like this.

The team who gets the highest score will win.

 

[Rover_KE]

I'd say The team that gets the highest score wins.

 

[teechar]

... to sing a song together on [STRIKE]the[/STRIKE] stage.

That's more natural if you're talking in general (not about a particular or previously mentioned stage).

 

[Tarheel]

I think the OP has a particular stage in mind. However, "the" in my opinion is still not necessary.