The present minimum salary grade level of public schoolteachers in the elementary and secondary school levels, regardless of their employment status, shall be upgraded to Salary Grade 18 per month: Provided, That the salary upgrading shall be differentiated in accordance with qualifications, skills, length of service and other logical bases of differentiation : Provided, further, That merit promotion on the basis of performance evaluation results shall be considered and treated as separate anddistinct from the upgraded salary provided for under this Act: Provided, further, That the salary upgrading shall not result to salary distortions, as defined under existing laws, within the Department of Education, Culture and Sports.