Starting September 2015, Stockbridge Community Schools stopped offering high school completion or GED preparatory classes. A+ English Language School offers some GED and ESL classes held in the old middle school library overseen by the Okemos School District. For more info, check out this flyer: adult-ed-classes-fall-2016-corrected
Here are some other programs in the vicinity that you may want to check out:
Lansing School District offers Adult Basic Education (ABE) and GED classes and GED Testing.
Potterville Public Schools offers ABE, GED and High School Completion,
Ann Arbor Public Schools offer a high school diploma program and GED classes for adults,
Washtenaw Community College offers the GED test,