Consider what help you need. Do you need help to keep the book filing system in order, to man the information desk or to help replace returned books? Create a plan of what staff you will need, and when. This makes the recruitment process much easier.
Advertise the positions. Include a brief description of what you are looking for, and approximately how many hours per week each volunteer is needed for. Include the advertisement in the weekly newsletter, and mention it on the school radio. Ask teachers to mention it to their classes, and display advertisements in the library.
Interview candidates. Create a list of the people who have expressed interest, and contact them to ask them which role they would like, and what they feel their skills are. Make notes about each candidate, and use these to match each candidate to a volunteer role.
Create a schedule. Ask the candidates to attend a meeting where they can meet each other, and hear about the role they have been given. Ask them to look at their school timetables, and write down when they are free to work. If possible, provide a list of shifts and allow each volunteer to claim some.
Train the volunteers. It may be possible to do this during school time by liaising with teachers, or you could individually train each volunteer at the start of their first shift. Ensure the volunteers feel comfortable doing their jobs before you leave them alone.