See Hawthorns Policy - Behaviour for policy on when exclusions may be made

Additional information on the processes surrounding exclusions and reviews of exclusions can be found here 
https://www.westsussex.gov.uk/education-children-and-families/schools-and-colleges/school-attendance-and-behaviour/exclusions/

The statutory guidance is here
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/641418/20170831_Exclusion_Stat_guidance_Web_version.pdf