Is it the case that your dd's school doesn't restrict attendance at these type of events to immediate family (i.e bio parents and siblings) only, chasegirl?
As HumblePotato said "... if this was 14 years from now and DD was graduating from Uni and the discussion was about whether or not OW should come to the ceremony then that woman would still be an OW.. She will carry that title for life IMHO".
I agree that, even where the term comes to be used in a quasi affectionate manner, OW (or WsofB) is what they are, and what they'll stay until they shake off this mortal coil.
As was said upthread, it's more likely the OW who negatively impacted on your life has insisted on being present, complete with flaking tan and piles, out of fear that her prize will gravitate to you ... as JG said, marrying a mistress creates a vacancy and it's ever the case that uneasy lies the head that wears the crown.
Adopting a magnanimous stance from the top of the compost heap moral high ground is de rigeur for public appearances of this nature but, as that doesn't mean you have to mingle up close and personal with the OW, you can be the serene countenance who is always on the other side of the room.
Dress in accordance with the occasion (smart-casual?) and reward yourself with a well-deserved
after you've given a stellar performance of 'am i bovvered' 
Good luck to your dd - I have no doubt she'll excel, as will you.
Update after the event please!