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 duplications you should only use the one record within Sims
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.