For year 11s who are going on study leave, they should be marked in registers with the S code only until they officially leave the school. The leaving date is input when completing Step 2 of the New School Year Setup process.
When completing this step, you'll need to add the leaving date in the future, which means you can complete the New School Year Setup before your year 11s actually leave the school.
You can see official guidance on page 19 of the DfE's School attendance main guidance.
In Arbor, you can use planned absences to add a pre-fill mark of S for all your year 11 students. You'll then need to open the registers each day for the marks to be pulled through to registers. You can see how to do this here: Do we have to open every register for marks to appear?