
As Lindsay says, just going into a class may be a bit much. What can help - what has helped a lot with Soli (who is frightened of dogs whether they are on or off lead, running, lying down, whatever) has been having a class where the trainer (who is a friend, luckily) has been aware of the problem and has been happy for me to work Soli a little bit apart from the normal class structure - so I've been focusing on her reactions to other dogs, moving her away if needed, moving her closer when she's ready, etc, etc. We've been able to join in the normal exercises too as allowances have been made to give her some space.
This same trainer does the same thing with any difficult dogs that come in - she didn't have much work with me as I was already working through the issues with Soli and knew what I needed to do, but with other owners she will keep an eye on them and send them away from the main group for the dog to calm down before it gets too upset.
That sort of class can be very helpful - just be sure to avoid the ones that have you punish the dog for reacting, that will only end badly.