The subject does agree with the verb because the subject is students and the verb is use. The subject is plural and the verb is singular.
Simple. I assume the ones going to the middle school are currently in 5th grade at the elementary school. You have 126 coming into the middle school from each school. You need to multiply 126 by 4. You get 504. There will be 504 new students at the middle school.
I Hope this helps.
If any farther questions lmk
Can u pls show what the story is bc nothing is showing
