If you're travelling on a school/college only bus, you will need a 16+ Travel Ticket to travel on it. It is not an option to pay for individual trips. If you're travelling on a regular service bus, it is an option to buy single trips, but this may be more expensive than buying a 16+ Travel Ticket.
You don't need a ticket to travel between one college/school site to the next as part of your course. Your college is responsible for providing the transport during the schooling day.
If you're over 15 years old and attending year 12 or a further education course, you do not need to buy a ticket. The Council must provide transport for everyone under 16 years old on 31 August.