For most teachers, vesting periods are largely symbolic. Merely vesting in a pension plan is not sufficient to guarantee a decent benefit, and many teachers would be better off withdrawing their contributions than waiting to collect a pension.
Most public school teachers are enrolled in a pension plan, but that doesn't mean they'll ever actually receive a pension, or that it will be a good one.
This post digs into worker retention data from Arkansas and notes that there are reasons to doubt how much pension plans affect worker retention rates.