Jumping To Conclusions

I'd say "encouraged by their thoroughness". It's totally plausible that a terrorist thinking of blowing up the bridge would ask questions under a different guise, and the most obvious guise is that of a student doing a school project. So it's a reasonable use of resources to check with the school that the story checks out.

Moreover, they're probably not spending all that much time tracking down students doing research projects, since no similar incidents were mentioned in the article.

Well, I don't know how plausible it is that a terrorist would do research by calling up the government and asking questions of them -- but if the government had infinite resources, I would agree that it's encouraging. But at a time when they ignore emails telling them that people have smuggled weapons through airport security, they clearly don't have the resources to be sending agents to visit middle schools. Couldn't this have been cleared up with a phone call?