No, never been invited to one & not sure I'd go. There is something to be said for moving on and not going back to revisit the past.
I did go back to my old secondary school 10 years after I'd left, to present the prizes at prixegiving. I'd not long got my PhD, and mum mentioned it to the head (who she used to see occasionally) and next thing I know, there's a letter in the post asking me to present the prizes. Funny thing was, I never won a prize when I was there. Caught up with some of my old teachers, which was odd, to say the least.
Although I can see the attraction of seeing what's become of them all (in some cases because I'm interested, in other to gloat over their decline) A few years after we'd left school I saw this chap I fancied rotten aged 14, he'd really badly gone to seed and I did think what a lucky escape.