Besides the head and body switching, some necks were stretched, one girl's arm was missing, and another girl's head was placed on what appeared to be a nude body, with the chest blurred.
A spokeswoman for Minnesota-based Lifetouch National School Studios Inc. said the alterations were "an unfortunate lapse in judgment" by an employee but didn't believe it was malicious.
The high school had required Lifetouch to make heads the same size and eyes at the same level in all student photos, company spokeswoman Sara Thurin Rollin said Saturday. The request was "unusual and definitely very particular, but that's not to suggest what happened here is acceptable," she said.
Rollin declined to say if the company fired or reprimanded the employee who altered the images. She said Lifetouch is taking full responsibility for the altered pictures, about 30 in all, and will pay to have the publication reprinted before the seniors graduate.
Lori Oglesbee, the school's yearbook adviser at McKinney High School, said the yearbook staff would spend the weekend rebuilding the yearbook.
McKinney is about 20 miles north of Dallas.
