NSES Content Standard Unifying Concepts and Processes: 
Constancy, change, and measurement 
Grades K-12, page 118 

Energy can be transferred and matter can be changed. Nevertheless, when measured, the sum of energy and matter in systems, and by extension in the universe, remains the same. 

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.

Benchmark 4E The Physical Setting: Energy Transformations
Grades 9-12, page 86
Whenever the amount of energy in one place or form diminishes, the amount in other places or forms increases by the same amount.

Benchmark 5E The Living Environment: Flow of Matter and Energy
Grades 6-8, page 120
Over a long time, matter is transferred from one organism to another repeatedly and between organisms and their physical environment. As in all material systems, the total amount of matter remains constant, even though its form and location change.

Benchmark 11C Common Themes: Constancy and Change
Grades 9-12, page 275
Along with the theory of atoms, the concept of the conservation of matter led to revolutionary advances in chemical science. The concept of conservation of energy is at the heart of advances in fields as diverse as the study of nuclear particles and the study of the origin of the universe.