The Mac's share of the lucrative back-to-school shopping season continued its upward trajectory this year, as data released on Wednesday indicates that some 26.8 percent of the personal computers sold in the U.S. between Independence Day and Labor Day bore Apple's logo.

Mac sales were up 14 percent year-over-year, compared to a 3 percent decline for Windows-based computers. Apple's MacBook line was particularly strong, according to market research firm NPD, up 16 percent over the year-ago period.

Google's Chrome OS also pushed ahead, adding 1.2 percentage points to finish with 4.5 percent of the market, a 32 percent year-over-year increase for the nascent web-centric platform.

Much of the non-Apple volume consisted of ultra-low-cost machines, the data suggests. Many Chromebooks — Â laptops running Chrome OS - come in at less than $300, and sales of sub-$300 Windows-based computers were up 37 percent with an average selling price of $242.

"After a slow start, aggressive pricing and robust selection drove significant volumes towards the end of the Back-to-School season, making it a very strong year," NPD executive Stephen Baker said in a release. "Due to the success of the aggressive Windows notebook pricing during Back-to-School we could see a much more aggressive pricing strategy this holiday season as the seemingly stable PC volume environment emboldens the PC OEMs and the OS and chip suppliers to make a grab for market share while the industry remains relatively steady."

Apple does not compete in the ultra-low-cost segment, making its ability to gobble up share even more impressive. The $899 11-inch MacBook Air is the company's least-costly offering, and the Mac's average selling price comes in well over $1,000.