Masada Armour created the bag, which costs upwards of £385, in response to the sheer number of shootings at American secondary schools in recent years.
Now, the firm says it is having to scale up production to keep up with orders from worried parents ahead of the new term.
“In two months we have sold hundreds and are gearing up to increase production rates to 500 units per month,” chief executive Snir Koren told the AFP news agency.
The backpack is designed for regular use but can withstand bullets from high-velocity rifles including AK-47s and M-16s.
It weighs five kilograms, meaning it is only suitable for older teenagers, but the company is now looking into creating a range for younger pupils too.
“We are developing a lighter model for their type of morphology,” said Mr Snir.
Requests for the backpack come as Betsy DeVos, the US education department secretary, said she would allow federal funds to be used to purchase guns for protecting schools.
The announcement outraged those who feel arming teachers – and essentially fighting fire with fire – will only lead to more deaths.
There have been two mass shootings at US schools this year alone.
In the first, Nikolas Cruz gunned down 17 people at Marjory Stoneman Douglas High in Florida in February. Three months later Dimitrios Pagourtzis murdered eight students and two teachers at Santa Fe High School, Texas.
There have been an additional 57 incidents of gunfire - at least nine of them fatal - since the start of the year, according to research carried out by the Every Town For Gun Safety Support Fund group.
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