https://www.irs.gov/pub/irs-pdf/p590a.pdf
They also include voluntary after-tax employee contributions to a tax-qualified retirement plan or section 403(b) annuity. For purposes of the credit, an employee contribution will be voluntary as long as it isn’t required as a condition of employment.
IRS Publication 590-A p.47
Employee contributions to my state's school employee retirement system (pension) are pre-tax and compulsory. Therefore, it does not appear to be a qualified plan. Getting a tax credit on pre-tax contributions is considered to be double dipping (i.e. two tax breaks on the same money) and is not allowed.
