Tonya,This sounds like a pretty fun project for students! Will there be some kind of prize for the group with the "best" bridge? How will you grade these projects? What makes you determine whether a bridge is good or bad? I feel the project needs a bit more depth for 7th graders. Possibly allow them to use some more advanced materials than just string and glue would make it more challenging and "real."
I'll endorse Elizabeth's comment. She said all that I had to say.