Meanwhile, more experienced substitutes are dropping off her list as they earn their own classrooms, Rosenberry said.
"So young people are not coming on and people with a little more experience are leaving us," Rosenberry said.
Hughes said for many, pay isn't the only consideration.
"The fact that our substitutes don't have any benefits ... That usually is a reason they'll go ahead and seek other employment," she said.
The nation's booming economy also plays a role, said Lynn Lerew, director of human resources for the Chambersburg Area School District.
It is an employees' market right now, and many substitutes are leaving to take available jobs in other fields, Lerew said.
"All employers everywhere are screaming for employees," he said. "They're looking for good people. People who want to work can find a (full-time) job."
As the pool of prospective substitute teachers shrinks, school districts find they are sharing those who remain.
Hughes said sharing with other school districts, coupled with the number of teachers on long-term leave, typically cut her list of 150 prospective substitutes in half on any given day.
Donna Newcomer-Coble, supervisor of human resources for Washington County schools, often has as many as 400 in her substitute pool to cover for approximately 1,000 teachers.
"But in terms of how many are active substitutes that we know we can get on a daily basis, we tend to max out at 80," she said.
School officials said that despite their best efforts, there is little they can do to ease the crunch.
The Chambersburg school district recently raised its substitute teachers' pay scale to make subbing more attractive, Lerew said.
Substitutes receive $70 a day, up from $65. When a substitute works in district classrooms for 45 days in a school year, the rate goes up to $75 a day.
Another way to compete is by installing automated "sub-finding" systems.
Teachers call the automated system to report they will be absent. The system then makes calls to find substitutes.
"It will work a 24-hour day seven days a week if you want it to," said Lerew, whose district is in its second year of using the SubFinder system. The Chambersburg system makes calls from 5:30 a.m. to 9:30 p.m.
Washington County schools also will report absences to a computerized system in the upcoming academic year.
"The substitute caller was having trouble making all the calls that needed to be made," said Deidre Shumaker, the principal of E. Russell Hicks Middle School in Hagerstown.
Newcomer-Coble said administrators hope to use the system to more accurately track the reasons both for teacher absences and for substitutes turning down work.
Meanwhile, the subs continue their flight toward full-time jobs.
Janice McClain, who had been a substitute for Pennsylvania's Greencastle-Antrim district, just took a full-time teaching position in that school system.
She said she looks forward to having a classroom of her own instead of receiving a 5:30 a.m. phone call and walking into an unknown situation nearly every day.
She said she hopes substitutes realize how important they are.
"Speaking from the other side of the fence, it's hard to leave," said McClain, who has held full-time teaching positions in other states.
"And you want to know that someone's coming in who will be good for the kids and who will teach them, not just watch them," she said.