It is very important that you arrive on time at school.
→ You must arrive on time at school.
It is very important that you arrive on time at school.
→ You must arrive on time at school.
“Very important that you...” = phải làm điều gì đó, nên dùng must.
Dịch: Việc bạn đến trường đúng giờ là rất quan trọng.