I'm an English teacher. Could you check my sentences for the students?

1. This river is the longest in Japan.
2. "Is Kyoto the oldest city in Japan?" "No, it isn't."
3. This flower is the most beautiful of the four.
4. Can Ben run (the) fastest in his class?
5. Jane is the tallest in her family.
6. This man is the most important in the group.
7. The first problem is the most difficult of the eight.
8. "Who is the tallest in your family?" "I am."
9. "Who came to school (the) earliest this morning?" "Mike did."
10. What is the largest lake in the world?

Thanks in advance.

Hirashin from Kyoto, Japan