
If you can find a trainer who will let you do your own thing, training classes can be absolutely fantastic for frightened dogs.
Thank about it - you have an area full of dogs under control, or on-lead even if they're not totally under control, with predictable behaviour in terms of moving around because the trainer is giving instructions.
That sort of setup was a godsend for Soli - in her case it was when I was attending my behaviour course at college, so not quite a training class, but all the dogs were on-lead and under control, with like-minded owners who were learning body language the same as I was so she found it much easier to cope with than dogs out and about. The same was true when we went to regular training classes during and after the course.
In those, the trainer knew about Soli (met her on the course, actually) and was quite happy for us to join in or not as I thought best - so I was able to spend each week doing lots of counter conditioning and desensitisation, knowing the dogs weren't going to come suddenly charging over or too close. And because it was a structured class, I could hear when the trainer said that recall was next, and if it was on/off lead, so could react accordingly.
The trick is finding a trainer that is happy with that setup, rather than one who wants to interfere or punish the dog for showing any signs of aggression - she had been through the same with one of her dogs so was perfectly happy for me to do whatever I wanted.