AC Transit says it would be saddled with too high a cost -- some $625,000 a year -- if it extended its youth discount bus pass to Oakland public school students 19 or older to accommodate refugees and special-education pupils who take longer to finish school.

In a letter approved Wednesday night, the AC Transit board said it is struggling to avoid further service cuts even without taking on a new cost to expand the youth pass currently limited to those 18 or younger. The district projects a $21 million budget deficit in June 2012 at the end of the next fiscal year.

"Without an alternative funding source and given our austerity mandate, AC Transit's deficit will increase and bus service will be cut even more if we extended our youth fare, which is based on age, to students over 19 years of age," the board said in the letter to Oakland school officials.

The AC Transit board also said it's sympathetic to the problem, and would work with school officials to find other ways of providing relief for older students who take AC Transit buses to school.

"We are going to work with them," AC Board President Elsa Ortiz said by phone Thursday. "But if we had to spend this money, we would have to cut service somewhere else."

This month, the Oakland Unified School District Board asked AC Transit to allow Oakland students 19 and older to be allowed to buy the $15 monthly youth pass instead of having to buy the $80-per-month adult pass.

The high public transit costs are a barrier to education, school and refugee advocates have said.

Among the older students are those in special-education classes, refugees, foster children, students with unstable housing and other at-risk youths, school officials said.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff. Read the Capricious Commuter at IBABuzz.com/transportation.