Actually, (and I've been embarrassed to say this for fear of getting flamed) I had bettas at my grad party. That's actually how I got back into them. I wanted something different for a centerpiece and I thought of bettas. I never knew they were used for weddings. I figured out I wanted to get 14 of them for the tables. They had small vases, that were bigger than their cups, and gravel. It was during the summer and in a building at the park so they would be plenty warm enough.
Now, I made sure I had homes for them BEFORE I had my grad party. None were "raffled off". I typed up care sheets myself and asked friends and family if they wanted them. I wouldn't have done this if I didn't have homes for them already prearranged. I even had most people pick out which ones they wanted before the party and i put tape with their name on it on the bottom of the bowl. I did have 2 red VTs leftover, but I figured I could care for them until I found someone to take them. Luckily, one of the caterers had a gf who was in love with bettas. She sounded to be really knowledgeable. So he took one and another caterer took the other.
It actually worked out nicely. They were only there for about 4 hours and none seemed too stressed. Everyone loved it and it kept kids occupied. I took every measure to make sure they were cared for and it worked.