Parent Lionel Bolton, from Cassington, wrote to the Witney MP after the county council deemed the A40 a safe road and announced it was cancelling free bus services taking children to the Bartholomew School in Eynsham after a review in April.
Mr Bolton, whose 10-year-old son is due to start at the school in September, said the road between Cassington and Eynsham was previously classified as dangerous and council figures show that 81 vehicle collisions have occurred on the road between 2010 and 2013.
He joined 30 parents and children on a protest walk on Thursday afternoon.
Five sets of parents, including Mr Bolton, have lodged an appeal against the decision and a hearing will take place on July 21.
Mr Cameron has written to county council leader Ian Hudspeth expressing an interest in the outcome of an appeal.
He also questioned why the previously “dangerous” road is now deemed safe.
Mr Bolton said: “I was very pleased when he gave us his support.”
A spokesman for the county council refused to speculate on whether Mr Cameron’s interest would have an impact on the appeal.
He said: “A number of families have appealed against the decision, and those appeals are due to be heard by an independent panel later this month.”
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