I thought the invitation letter is given by your school to
you on their letter pad. So they must have already had the template. All you
need to do is give them the info like

passport number, Name, DOB etc. Verify the information on
the letters is correct once you get it and then send it to you parents.

Here is a sample if you still need it
http://oiss.rutgers.edu/pdfs/Forms/General%20Forms/graduation_inviting%20guests.pdf