In your 'Save' microflow you can add functionality, including the possibility to create and connect a Student object. If you have a default Save button, you can change this to a custom Save action, in which you call a microflow to perform these actions.
EDIT: added screens:
In the popup for adding users, you can right-click the save button and select 'go to microflow' to see the functionality that is being triggered when clicking this button.
In this microflow you can add the 'create object' activity, in which you can create a Student object, that you can link to your account object. In order to do this, you need to add this association in your domain model (link the account object to the student object)
Hope this helps!