I have a six month old bernese puppy who was walking well on a lead until I went to a terrible dog training class. There she became frustrated and started using her lead as a tug toy. Since then the problem escalated and she is now impossible to walk on a lead without her grabbing the lead in her mouth and trying to play tug. She is clicker trained and so I have tried clicking her every time she drops the lead, but even with chicken as a reward she would far rather try to play tug. I have bought new tug toys and tried to play loads of tug games before putting her on a lead. I have tried borrowing a chain metal lead, but that hasn't helped. Does anyone have any suggestions as to how I can stop this behaviour and get her to walk nicely again?

How did you teach her lead-walking in the begining, I take it you clicker trained so there was use of rewards? If so, go back to what you did before, but swap the previous rewards for a game of tuggy on her tug-toy.
Seems like the most 'rewarding' thing for her when it comes to tuggy with the lead, IS a game of tuggy, so use her tuggy-craving to your advantage :)

Change the lead to a chain one whilst you are training her to tug on command on specific items to interrupt this behaviour.

"How did you teach her lead-walking in the begining?"
Yes - I clicked when she was walking nicely by my side and gave her a piece of chicken/cheese after every click.

Playing with a tug-toy when she walks nicely is a good idea. I'll give it a try.

I also like the idea of training the 'tug' command. *heads off into the garden to give it a try*

Thanks for the ideas!

It's very useful to have a dog that will tug on the lead as it can be used as a reward -and the lead you always have with you. You will see plenty of agility people doing it for instance. As others have said, teach to tug and stop on command -using a tuggy as opposed to the lead initially.

A word of caution here. I used to do some private training years ago and had a GSD adult about 2 years old who did just this. It was not a problem when she was young and she would soon drop it to look at something more interesting as youngsters do. As she got older she gradually got more and more fixated on holding the lead and would grip it tightly if the owners tried to take it out of her mouth. By the time they came to me it was a huge problem and she would growl and refuse to release it and it made walking her very hard.
It took a long time and a lot of patience and treats to get her to a stage where she would swop it for a ball. We eventually got her so that she would go for her walks carrying her ball and drop it on command. The owners came close to having her PTS as they thought she would never get better and she was getting quite agressive about it.
I would work very hard to stop this behaviour as soon as possible. Better to have a tuggy to play with and keep leads for the purpose they are made for.
Aileen

A lot of agility folk have a tuggy that they attach to their lead and this is used as a reward. They tend to use the soft fleecy type of tuggy and the dogs do know the difference between the lead and the tuggy.

Use 2 leads the second she grabs 1 you drop it and use the other!! She gets to carry something in case that is what she wants, but if it's for the thrill of the game, it stops and you still continue good heel work!

Works for me everytime, I use a lead as a reward as wellbut only when we have strict rules of how to play in, take it leave it/stop etc. 

drench a cloth lead in bitter apple.