There might be a situation where current students are also employed by the School or the student has left and come back as Staff. To avoid a duplication, you should only use the one record within Sims. To do this, go to:
Focus > Person > Staff > New
Type the Surname of the Person and click Continue
This will then bring up a list of matched people.
You need to highlight the record that says Student under the Roles and Click Open (if you Click New this will create a duplicate record)
This will Open a New Staff record for your Student and complete the Personnel details as normal.