I suppose it really depends on how close your friendship is going to be with these JWs. At the moment they are just your students and have no right to know anything about your private life. All that concerns them right now is that you are qualified to teach them.
If you start meeting them socially then I suppose you might feel obliged to tell them at some point, but I would be as casual as possible and just say it is part of your past life, that you respect their beliefs but would rather not talk about it if they didn't mind.
I am still on friendly speaking terms with almost every JW I know even though I DA'd over 20 years ago, we just keep off the subject of religion lol. There will always be the odd few fanatical JWs who think you have sprouted invisible horns and there are demons dancing round you
I would deffinately keep it to yourself for the time being until you have got to know them more as individuals so you will get a better idea of how they will react to knowing you are DA'd. Please don't worry too much about it.