Please find below a link to the Arriva web site, where you can purchase bus passes for a variety of timescales. An annual pass, is approx £600 (vs approx £1000 for the school bus service into Aylesbury for the 16yrs+ age group).
The pass is valid on any bus, at any time, including weekends, evenings and school holidays. So may therefore help your 6th form student reach weekend work and leisure activities as well as school/college etc and provide a viable alternative to the dedicated school bus service. Do check bus times though.
Please direct any queries re the cost of school bus travel direct to Bucks County Council.