Experience makes better teachers...(of course it does)
"...said Jeremey Ayers, a policy analyst at the Alliance for Excellent Education, but there is solid research showing that when teachers have at least five years of experience, students do better." AP April 7, 2007, States Try Mentoring To Retain Teachers.
"...many studies have established that inexperienced teachers (those
with less than three years of experience)are typically less effective than more senior teachers...." Teacher Quality and Student Achievement: A Review of State Policy Evidence (open PDF file here to view), Linda Darling-Hammond, Stanford University, December 1999, page 9.
Keep experienced teachers, offer a rising pay scale.