The boy, who is not named, was expelled two terms before his GCSE exams after he and friends connected up a phone during the lunch break and he impersonated his form master during a call to his home.
His mother says the punishment is out of all proportion to the offence. Her son, who had not been in trouble before, has had to switch GCSE syllabuses at the last moment and to take only eight instead of ten subjects.
She said: 'I was told I could appeal to the chairman of the governors but as he was party to the decision there was little point.
'It is ironic that the more you pay for your children's education, the less input you have. The state system is riddled with safeguards.'
Keith Hampson, Conservative MP for Leeds North-West, said yesterday: 'This doesn't seem an appropriate punishment . . . Independent schools should set up an appeals panel.'
Derek Fatchett, Labour MP for Leeds Central, said the case, which happened last November, was outrageous. The boy's exam results had been jeopardised. The Government said in answer to his parliamentary question that private school parents could not be given more rights because they had entered a private, commercial contract.