A school dinner lady is claiming damages from a teenager who seriously injured her while playing tag with a friend. If she is successful, pupils could become legally liable for boisterous behaviour that results in accidents.   Michelle Orchard, 42, is seeking hundreds of thousands of pounds in damages from Sebastian Lee, who was 13 when he ran into her during the lunch break at Corfe Hills School, Corfe Mullen, Dorset, in January 2004.   Sebastian, with his friend in pursuit, tumbled into her backwards, striking her in the face with the back of his head, senior judges at the Court of Appeal heard yesterday. Mrs Orchard's nose was broken and her teeth were damaged. Her lawyers say that she also suffered serious brain damage, leading to partial paralysis and balance problems. She has been unable to work since the accident.