Lorraine Godden, from Hampden Park, says her son, Lee Ashton, a pupil at Causeway School, is being home-educated because East Sussex County Council will not pay travel expenses to his school.
Lee used to get the bus from outside his house to the school gates but when the route was discontinued and replaced by a school loop bus, Lee twice became confused and ended up lost in the town centre.
Lee's mum, Mrs Godden, said, "Lee is very happy at Causeway School and the headteacher, Terry Boatwright, and his staff have been really supportive in trying to help Lee stay at Causeway.
"However, though we've been through the tribunal process with the county council and an appeal, they have decided they are not prepared to pay for Lee's taxi fare to and from Causeway School.
"We simply cannot afford to pay ourselves and because of Lee's disability we dare not risk letting him make his own way by bus to the school. The last time we tried it he ended up in the town centre very confused and frightened.
"The education department know that my own disability also prevents me from taking Lee to school myself so I am at my wits' end about what to do."
She added, "Causeway School has a really good special needs department so I do want him to attend but County Hall just doesn't appear to be interested in helping us."
Lib Dem parliamentary candidate Stephen Lloyd, who has been trying to assist the family, said, "I find it incredible that despite all the necessary evidence County Hall does not want to help this family.
"It's also not good enough for the county council to say that Lee can go to ETC as his siblings attend Causeway. His learning disability means that changing school would be very hard on the lad and the whole family feel very strongly that the best education available for Lee would be at the school he has got used to.
"I will be writing to the cabinet member responsible for education in East Sussex asking that he urgently intercede on Lee's behalf."
An East Sussex County Council spokesman said, "We are concerned to hear that Lee is not attending school.
"We have a very clear policy which is designed to be fair to all and which sets out who is eligible for transport support for getting to school.
"In general, the eligibility relates to how far away they live from their local school. There are some young people, however, whose disabilities are such that they require special support, for example where a child is not able to walk. These are exceptional circumstances.
"We have assessed Lee's needs and have concluded he does not meet the criteria for support under our policy. That decision has been considered and upheld on two occasions by a panel of elected members."