It can backfire though as my little brother found out. After regualarly having monday morning "tummy ache" my Mum had enough (the pain usually disappeared about 10am) and told him to go to school but come home if he felt worse, about 2 hours later she collected him from school (he actually looked ill now) visited the GP who diagnosed appendicitis, so she took him to hospital where he had an appendectomy. Mum felt guilty for ages but it taught him a valuable lesson! I always used headaches as my excuse, also impossible to disprove...