Often students have really great written English and could understand the magna carta when it's written down... but speaking and listening... phew, it's a whole different story! Which only gets harder when you stir in large class sizes AND mixed abilities!
So, first up, the large class problem... there's some good advice here which might come in handy for you.
Next, the issue of different levels. Bit trickier this one: If you have different levels in the classroom and with a group this big, I would set a short writing task, and when you are marking it make a note of strong, medium and weak ‘groups’. You can then focus on a few students every lesson and asses them and modify the list as necessary. At times, you may want to put students into groups with students of similar levels to work on different skills – so you can use your list to help you do this. Also sometimes you may wish to set the students off on an activity and go to help the weaker students (which will be easier if they are sat together!). Also sometimes it is better to pair stronger students with weaker students so they can help each other. It’s best to keep changing the groups however.
One of the best pieces of advice I’d give for control of the classroom is to put students into groups and every lesson give/take away points to their allocated group for good/bad behaviour, doing/not doing homework, using English/ not using English etc.
For speaking activities, students always speak more if they have something in their hands such as a ‘find someone who’. If you want students to create role plays, provide a model and then ask them to work in groups of 2/3 – then they can perform to another group after. But remember to give them role play cards! http://www.learnenglish.de/Teachers/roleplays.htm. You could write this on the board if you can’t photocopy materials! Another website is http://www.esl-galaxy.com/speaking.html with some conversation materials.
Or you could create a gap filled role play that they have to fill in and then perform. (If students are shy then this could be a better option). Also think about drilling for pronunciation key sentences. If they practice with you first (do it dramatically!) then they will perhaps feel more comfortable!
For more advice on role plays see http://serc.carleton.edu/introgeo/roleplaying/howto.html and http://esl.about.com/od/intermediatepronunciation/Pronunciation_and_Speaking_Skills_for_Intermediate_Level_English.htm
You might want to create your own role plays based on what your students like – interviewing popstars? Talking about sports? Shopping? Here is an example of a role play: http://www.esl-galaxy.com/speaking/Shopping%20for%20a%20present.pdf
Has anyone else got any good ideas for conversation classes? Let me know all about them!