I wouldn't give anything and wouldn't even admit to anyone that I actually got the invite if a follow up was made. That's just a gift grab of epic proportions.
Ditto. You had to work at figuring out who the girl is -- and you're still not certain? Nothing.
People just call them announcements, not sure why.
An invitation and an announcement aren't the same thing:
An formal invitation is worded "_____ invites you to ______ event."
An announcement is worded "______ announces _______ event."
Some schools hold BIG graduations and students are allowed to bring as many guests as they like. Other schools -- usually because of venue -- hold much smaller events, and students are only allowed 2-3 guests each. Thus, schools tend to provide "announcements" because they aren't really inviting everyone. We'll send them only to family and close friends who'll actually attend the party -- and I won't be insulted if they don't want to go to the graduation itself. Ours is a HUGE circus of an event, and parking is difficult.
Technically an annoucement is mailed on THE DAY OF THE EVENT so people do not mistake it for an invitation. For example, some people who are getting married out of town (and, thus, aren't inviting anyone except immediate family) send announcements so people know they've been married. However, people tend not to recognize the difference between invitations/announcements today, and graduates tend to send announcements more on an invitation-timeline, meaning several weeks ahead of time; of course, this further blurrs the line between invitations/annoucements.
Announcements never require a gift -- they're just informing a person of an event.
Invitations are a little more "serious", are usually sent only to friends who'd really attend, and usually mean a gift.
Personally . . . when my first graduates next year, we'll send out homemade cards -- anyone who knows me would expect that -- that'll give her graduation date/time and invite people to a lunch (maybe dinner -- don't know the time yet) on the same day. So it'll really be an invitation. We will not buy the school's butt-ugly announcements.