Yet another familiar school-days object may be going the way of the inkwell and the slide rule.

Encouraged by a milk industry study that shows children drink more dairy when it comes in round plastic bottles, a growing number of schools are ditching those clumsy paper half-pint cartons many people grew up with.

More than 1,250 schools already have switched to single-serving bottles. While that is still a tiny fraction of the nation’s schools, it is a significant jump from 2000, when there were none, according to the National Dairy Council.

“Those damn square containers are awfully hard for kids,” said New Hampshire Agriculture Commissioner Steve Taylor, who has watched the trend spread to some 320 schools in New England. “Teachers say you can spend the whole lunch period just walking around and opening those containers.”

In Lawrence public schools, elementary students still drink milk from 8-ounce cartons. In junior high and high schools, both cartons and plastic bottles are offered in breakfast and lunch lines.

Next week, district crews will install milk vending machines at Lawrence and Free State high schools — offering 12-ounce bottles of regular and chocolate milk in skim and low-fat varieties, plus strawberry milk and orange juice.

Officials hope that the increased availability will lead to more sales.

“Kids tell you what they want,” said Paula Murrish, the district’s director of food services. “My biggest fear is it doesn’t sell and it expires.”

Industry studies indicate she has little reason for worry.

Though plastic long has been the favored packaging for soda and other drinks, schools sought bottled milk after a 2002 Dairy Council study found milk consumption increased 18 percent in schools that tested bottles. The study also found that children who drank bottled milk finished more of it.

Students show off their choice for milk containers at lunch in Woddbury Middle School in Salem, N.H. More schools are ditching half-pint cartons for single-serving plastic bottles after a milk industry study showed children drink more dairy when it comes in plastic bottles.

The change to plastic brings schools closer to overall milk packaging trends. In 2001, more than 82 percent of the nation’s milk was packaged in plastic, up from 15 percent in 1971, according to the U.S. Department of Agriculture.

While the growing use of bottles in schools can partly be attributed to ease — educators say plastic caps are easier for children to open, and round bottles fit better in their hands — marketing savvy deserves at least as much credit.

Several years ago the milk industry decided its boxes were not visually competitive when sold alongside the relatively sexy bottles of juice and soda increasingly common in schools.

Now, many schools display the bottles in glass-front upright coolers — just like at the convenience store — and obesity concerns have prompted schools around the nation to oust soda machines in favor of milk vending machines.

Fast-food chains Wendy’s and McDonald’s recently replaced their milk cartons with bottles and sales soared.

Bottles also could be a financial boon for school lunch programs, which depend on meal sales to stay afloat.

Though bottled milk costs schools more, high schools that served it during the study saw lunch program participation increase nearly 5 percent, said Grant Prentice, executive vice president of marketing for the Dairy Council.

Jeanette Kimbell, food service director for schools in Nashua, N.H., tried bottled milk at the city’s three middle schools last year after earlier efforts — including offering milkshakes — had failed to get children to drink more milk.

Now the district’s 3,300 middle school children are drinking 10 percent more, and they are telling Kimbell bottled milk tastes better. And lunch program participation is up between 8 percent and 18 percent at each of the schools.

Because of the rise in lunch sales, the 11 cents more per serving paid for bottles is not passed on to the children, Kimbell said.

The milk industry also is likely to benefit. Americans have been drinking less and less milk since the 1970s; dairy officials hope reversing that trend among children will result in a lifetime of drinking more milk.

There also is potential hidden growth for the $11 billion milk industry. Schoolchildren consumed 5.3 billion half-pint servings of milk 2002. But many of the new bottles hold 10 ounces, or 2 ounces more than a half-pint.

For dairy processors, changing over to plastic can be costly.

Bruce Matson, spokesman for the Maryland and Virginia Milk Producers Cooperative based in Reston, Va., said his company invested more than $1 million about four years ago to change a third of its bottling business to plastic.

So far it has been worth it. Schools that made the switch now are buying 20 percent more milk, Matson said.