Benchmark 4D
The Physical Setting: Structure of Matter
Grades 6-8, page 79

No matter how substances within a closed system interact with one another, or how they combine or break apart, the total weight of the system remains the same. The idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same.

NSES Content Standard B 
Physical Science: Properties and changes of properties in matter 
Grades 5-8, page 154 
Substances react chemically in characteristic ways with other substances to form new substances (compounds) with different characteristic properties. In chemical reactions, the total mass is conserved. Substances often are placed in categories or groups if they react in similar ways; metals is an example of such a group. 

See also the general discussion in Unifying Concepts and Processes: Constancy, change, and measurement