Fortigate 300E - Captive Portal auth timeout

I work in a school and I have the following scenario: In computer labs, I need to know which students are accessing the internet throughout the day, and for that I am using the LDAP base of our institution. Creating a captive portal, pointing to this LDAP, the registry worked ok. However, I need the session to expire in a few minutes, to ensure that a single student is not responsible for accessing the internet from a particular computer throughout the day. For this, I configured User & Device -> Authentication Settings -> Authentication Timeout in 40 minutes, but this did not have the desired effect, the sessions remain connected after the 40 minute period. Could you help me solve this? Sorry for the English if it is incorrect, it was generated by google translator.