Need an answer quick!

Call your vet quick, and ask them if they'll do a fecal quick for you.  This honestly shouldn't require the vet's input UNLESS there's something wrong.  They may be able to get a sample with a rod or if you can pick up even a partial sample with a plastic baggie -- you can get them to look at it while you wait usually.

 If it were ME who had class with you tonight, I would honestly hope you'd make sure the dog doesn't have parasites or a bacterial infection BEFORE you bring him around other dogs.  Potentially  a waste, maybe -- but it's a point of responsibility not to bring your animal somewhere like that if you even suspect there could be a problem.

Stress, drinking from a puddle -- even such simple stuff can cause bacterial overgrowth in the gut, and usually that requires an antibiotic/anti-inflammatory for the gut like Flagyl (metranydazole) -- not serious but it needs to be treated. 

 Sorry -- not the answer you really wanted but ... it's my take anyway.

I agree with Callie (as usual).

I have gone to class without my dog before, when he was ill or hurt.  That way I get to keep learning and can get some practise in before the next class.  That way you still get what you have paid for but you are not stressing your pupper or endangering other dogs (depending on what is wrong).