but I think his total awkwardness around Lana that day is supposed to be why he's moping around on the bridge.

I always thought his moping on the bridge was due to the fact that his father wouldn't let him joing the football team. He was forced to watch Pete practice, and even though he had his little lana fantasy while there, he still would have been there waiting on Pete, as the reason for going out for the team was to save themselves from become the scarecrow. Clark's being down and hanging out on the bridge, imo, is due to his failure to do something (join the team) that would make him feel like a normal teenager -- Lana here or there.