Dear (your friend's name),
(Your name) is pleased to accept your kind invitation to attend the graduation party of your daughter (daughter's name) on (day/date of party)
at (time of party).
Or, less formally.......
Thanks for inviting me to (daughter's name)'s graduation party on (date) at (time). I'm very pleased to accept your kind invitation and look forward to seeing you then.
buggles(not a teacher)