Out of the past ten or so races, the majority of the bike numbers I've used are wraparound sticker-types, with the number printed twice, back-to-back, so that when you stick it around one of the tubes it displays on both sides. You either deal with it flipping on your knee once in a while and wrap it on the top tube, or (as I do, because I have a compact frame and the seat tube's pretty high in relation to the top tube) wrap it around the seat tube or supporting forks. I found a pic: flagrantly googled.

I had one race with the paper/ziptie or twistie tie number, and the thing got wet after I spilled water/sweat on it, and fell off. Most people's did, and anyway, you have your race number written on your arm, your helmet number (usually) stuck on your helmet, and a lot of people clip on their race belt with their running number on before they even hop on the bike - saves time in T2.

In summary, I wouldn't worry about your bike number for St. A's. If it falls off, they get to clean it up, and it's a huge race, so I bet you get a sticker. I would, however, invest in or borrow a race belt - safety pins are for 5k's!

Have fun and GO TEAM!