Sure, here are the rewritten sentences:

1. Phong can't decide who he should consult about his problem.
→ Phong can't decide whom to consult about his problem.

2. Do you have good relationships with your classmates?
→ Do you get along well with your classmates?

3. The exam is difficult. The students become hard-working.
→ The more difficult the exam is, the more hard-working the students become.

4. Do regular exercise and you may have good sleep.
→ If you do regular exercise, you may have good sleep.

5. Giving priority to work, school, and family is really difficult for students.
→ It is really difficult for students to give priority to work, school, and family.