[fjm]

It sounds to me as if class is just too exciting and different and a bit bothersome for him to be able to concentrate on you, and he has not yet learned to generalise the cues for Sit etc. "Sit!" means sit down on the rug in the nice safe sitting room at home - it does not yet mean sit wherever you are, and whatever is happening around you. To teach that you have to teach it in lots of places and circumstances, gradually working up from the only very slightly distracting to the full blown overwhelming stimulation of a first training class.

I would forget about trying to force him to sit, and just get down on the floor beside him in class and play Look at That! for a while (search on here for how to play the game). When he is able to relax around all the other dogs and people he will be able to give some of his mind to learning cues. Meanwhile work on generalising the cues he already "knows", starting in your own yard, then on rather boring walks, and in gradually increasingly distracting circumstances.

You will hear dog trainers talk of the 3Ds - Distance, Duration and Distraction - each needing to be worked on separately and then together. I found Jean Donaldson's book and dvd "Train your dog like a pro" a helpful step-by-step guide to what this means - many of the videos are online, but the book has detailed "recipes" that you may find useful.