When I worked in schools, we asked for contributions and suggestions from student representatives, members of staff, as well as members from the senior leadership team. Student voice was really effective. They have loads of ideas to offer and will have varying experiences and backgrounds. You can hold meetings with students and staff once or twice a year and use a random name generator to ensure that people are picked at random to keep the system fair. By discussing policy ideas with groups of students from different year groups, and a range of different subjects, you can get insight into a lot of different perspectives and values that underpin school life. This will help your policies to become more inclusive.