During the latest incident, a 16-year-old girl was on her way to College Park High School about 7:30 a.m. Tuesday when a man sneaked up from behind, grabbed her buttocks and fled at a full sprint, according to police and district officials.
The 11th-grade girl told officers that the incident took place near Harriet Drive and Violet Way.
The girl snapped a photo of the man as he ran away, police said. He was last seen running southbound on Shirley Drive from Harriet Drive.
The suspect was described as a Hispanic man in his 40s. He wore a blue LA Dodgers windbreaker over a gray, hooded sweatshirt and bluejeans, police said.
The suspect's description differs from a similar incident reported Monday morning in the same area.
In that assault, a 17-year-old girl told police that a man inappropriately touched her as she walked to school on Monday at about 7:40 a.m., police said.
That suspect was described as a 30-year-old white man, about 5 feet 6 inches tall, with a thin build and short sandy-colored hair, officials said. He wore a white T-shirt, tan jacket, sunglasses and a backpack.
The girl was on her way to Prospect High continuation school when the man sneaked up from behind her, touched her and ran off, according to police and district officials.
She was on a popular walking trail between W. Hookston Road and Astrid Drive, about a mile from the continuation school, which shares a campus with Pleasant Hill Middle School.
"We're hoping we can do whatever we can to find out who this is," Mt. Diablo Unified School District Superintendent Nellie Meyer said following the reported assault.
Meyer said principals at both schools sent out phone messages to families following both incidents.
School officials are encouraging students to pay attention to what is going on around them while walking to and from school, instead of using cellphones, Meyer said.
Anyone who has information about either incident is asked to call police at 925-288-4600.
Theresa Harrington and David DeBolt contributed to this report. Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund.