You can toggle auto-accept on or off through your session settings.
You can go to your Calendar page, then click on the small gear icon on the upper-right-hand side of the session card you want to toggle auto-accept on/off.
Then click on Edit Session.
Then go to Additional options > Personalise your session > Auto-accept session requests
You have the option of toggling the auto-accepting session requests for new connections (mentees) and existing connections (mentees)