24 Pa. Stat. § 22-2202
The board of school directors of any district may provide, in the contracts with its teachers, principals, or supervising officials, that they shall contribute a reasonable sum from their salaries each year to said retirement fund. No person shall be required to contribute any part of his salary to any retirement fund, unless the same is provided for in the contract by which he is engaged.
The board of public education of any district of the first class may provide, in the contracts with its employes that they shall contribute a reasonable sum from their salaries each year to said retirement fund. No employe shall be required to contribute any part of his salary to the retirement fund, unless the same is provided for in the agreement by which he is engaged.
24 P.S. § 22-2202