Hundreds of students from Waltham, Watertown and Newton got a jump on their back-to-school prep Monday — boosted by the Salvation Army’s program to distribute 850 backpacks full of supplies.

Active-duty U.S. military members and their accompanying spouse and/or children traveling on Spirit get two free checked bags and a free carry-on, in addition to the already free personal item.

For many military families, summer can be stressful, as they are often relocating to different duty stations. Operation Homefront, the national nonprofit campaign has been distributing backpacks ...