In a large city school system with 20 elementary schools, the school board is considering the adoption of a new policy that would require elementary students to pass a test in order to be promoted to the next grade. The PTA wants to find out whether parents agree with this plan. The following idea for gathering data has been proposed.

Run a poll on the local TV news, asking people to dial one of two phone numbers to indicate whether they favor or oppose the plan.

What kind of sampling strategy is involved? What (if any) sources of potential bias do you see?