That's a question I used to hear a LOT! Here's an easy solution that's helped me tremendously in not only collecting student's wet paintings, but also in distribution during the next class.
Each table has a table number (you probably already have a similar system in place).
I put tabs on my drying rack that corresponds to the table numbers (colors, whatever...) over the tables. This way, the paintings are in order for a quick distribution the next time the kids come to art!
I also keep a drawer of clips close by with the class codes on them. They are placed on the top of the drying rack so that the kids can confirm that they are putting their paintings in the right spot.
Hope this helps someone! :)