Hi Brigitte
You have been given sage advice above on setting up your leavers’ database. Personal contact details are essential!
In years to come you may want to ask for connections to Past Pupils ie if their mother/father went to the school.
I work at an all-girls school and my biggest bugbear is maiden names/married names! You could make a point of asking your leavers to notify you if they get married and change their surname (although marriage will seem like a long way off, they may remember to notify you when the time comes.)
Good luck with your exciting opportunities.
– Mary