Final answer:
The legal basis for increasing teachers' starting salary to P18,000 a month is under RA 9155, the Governance of Basic Education Act of 2001.
Step-by-step explanation:
There is a legal basis for increasing the teachers' starting salary to P18,000 a month under RA 9155, also known as the Governance of Basic Education Act of 2001. This law mandates the Department of Education (DepEd) to establish a compensation and position classification system for teachers and allows for the determination of salaries based on qualifications, experience, or other relevant factors.