To run this go to Reporting > Reports > Students (tab) > Special Cohorts.
In this report enter the Date Range you wish to look at mobility for and select a Filter to run the report. For example Whole School if you want to see mobility at a whole school level or year if you want it broken down by year. Then click Choose to run the report.
You'll then see the mobility in and mobility out columns.
To view this information in more detail you can use the Admissions and Leavers Report.