Sunday school

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A school, generally affiliated with a church or synagogue, that offers religious instruction for children on Sundays.
• noun The teachers and pupils of such a school.

from The Century Dictionary.

• noun A school for religious instruction on Sunday, more particularly the instruction of children and youth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A school, organized by a church, that provides religious education to children on Sundays.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun school meeting on Sundays for religious instruction