San Francisco authorities are searching for a pair of boys who escaped from juvenile hall last night.  KTVU says the 12 and 16-year-olds smashed a glass window, and were able to squeeze through the small opening before scaling a 20-foot barbed wire fence.  The boys were reported missing just before 10 p.m. at the Youth Guidance Center on Woodside Avenue.  The parents of the two escapees have been notified.