I worked as a TA when I was straight out of uni and had some sixth formers saying things about me in my hearing (I was overweight) and taking the mickey in general. I don't think they saw me as a member of staff, as I looked young and didn't have any kind of intimidating ability! So on your placements, with any luck you will be introduced as a new teacher and the children should see you as such. It won't stop them from talking if they want to, but hopefully it will prevent them from doing it in your hearing. As you are now, a visitor essentially, the children aren't thinking about giving you the level of respect they should, because you aren't a teacher. Sad, but possibly true. Another thing I would suggest is to perhaps start off in each new class with a discussion about your lisp; bring it to attention and perhaps allow them to ask questions. You may find that being open about it from the beginning shows you have nothing to be embarrassed about and this should rob them of the chance to say anything mean. Sorry, just some ideas. Perhaps more experienced teachers can give other suggestions.