A former Lancaster County high school teacher was sentenced this week after pleading guilty to storing child sexual abuse material on his phone and sending nude images of himself to undercover investigators posing as a 14-year-old girl, the county district attorney’s office said.On Aug. 31, David S. Burkhart, 45, of Warwick Township, pleaded guilty to three counts of sexually abusing children, two counts of criminal use of a communication facili…