To be honest, Winston's behaviour is fair enough but of course you don't want him to be practising it I think it is up to the trainer to help you work on this within this environment BUT she needs to make it easy for you both.

For example do you all sit in the same places each week, and how are the seats arranged? I'd ensure you are placed well away with preferably a good distance between you, and also a good supply of extra tasty food treats to help with training - use really special stuff for this.

Ensure the other dog cannot eyeball yours - this is not alwalys noticed even by trainers, but you can ensure this doesn't happen by keeping your boy turned away from the other dog whilst he is kicking off

He's starting to "generalise" - the next step may be to do this with all dogs, or all medium size dogs of the same colour... hence the importance of setting up the environment so that he cannot practice.
If it continues for one more session, with no improvement or knowledgeable help for your dog/situatoin, I'd leave, yes. It is not worth messing up your dog, which it *could* do. It might not, but it's not worth the risk IMO.

If you see spaniel type dogs out and about, have tasty food ready and generously give to your boy (do not get so close that he reacts..) and stop the food when the other dogs go past/move away. This will help him to build good assocations with dogs that look like this dog.


Wys
x

The trainer asked me today at my other dogs agility class how I felt about it. I let her know about Winstons reaction when out walking etc. She is going to do some socialisation with new dogs with him after tomorrows class to show him that not all new dogs are going to bark at him.

She also told me she has been discussing with the other trainer what to do and have decided that if the other dog doesnt start responding then they will have to ask him to leave. (this isnt the first class he has been disruptive in)

If you get edgy then Winston will pick up on this. Try and stay as relaxed and as confident as you can at the class and also when out and about when you see springers. Hope everything is resolved soon. If he insists on barking at strange springers just make him sit and talk to him/distract him until they have gone. That springer at the class sounds out of control and must be quite a distruption.
Good luck.