Maryland Code, Education § 9-108

(a) Employees of a public charter school:

(b) If a collective bargaining agreement under Title 6, Subtitle 4 or Subtitle 5 of this article is already in existence in the county where a public charter school is located, the employee organization, public school employer, and the public charter school may mutually agree to negotiate amendments to the existing agreement to address the needs of the particular public charter school, including amendments to work days, work hours, school year, procedures for transfers that are consistent with the instructional mission of the school, and extra duty assignments.