Democratic lawmakers are scheduled to unveil their own plan to evaluate Maine’s public schools. The plan expected to be unveiled Wednesday is an alternative to Republican Gov. Paul LePage’s A through F system revealed last week that was criticized as overly simplistic and biased against poorer school districts. The Democratic co-chairs of the Legislature’s Education Committee, Sen. Rebecca Millett of Cape Elizabeth and Rep. Bruce MacDonald of Boothbay, are proposing a bill with “at least a dozen” criteria for evaluating schools. A spokeswoman for Senate Democrats says the system is based on student progress and local improvement measures. A spokeswoman for LePage says it’s late in the game to introduce a new system, and changes are already being implemented based on the letter grading system.