Having tried many different ways to pin down classroom teachers I find the most successful strategy is get on their class or grade level newsletter mailing list. It may be a website, a pdf sent via email or the old-fashioned printed variety but almost always there is something sent home to keep the families informed.
This does not take the place of actual planning with the teacher for collaborative lessons but it will help you know what the kids will be requesting when they come to the library that week.
thanks, seems like I'm hearing (and knew in my heart) that collaboration cn be a one way street. I wish sometimes they would realize that we don't just "know" we have to be informed.