As a teacher it is your responsibility to be able to cater the work for all levels of abilities within the class. If you have a range of ages then you may especially find that there is a vast range of abilities too. You need to be able to create one lesson that allows children to interact at different levels. If the work is too easy then the children will get bored and start to play up, if the work is too hard they may give up trying. Teaching mixed abilities can make lesson planning a little bit more difficult as you need to ensure that there is work for everyone at the correct level.
You may decide to split the class in to smaller groups to allow you to give each group a separate task and possibly sit the teaching assistant with the lower ability group to give them a little bit more assistance.
Another good idea is to ask the pupils to work in pairs, putting someone that is at a higher ability level with someone that has a lower level to allow the pupils to learn from another. Always make sure when doing this that the work is being done by both students though and not just left up to the one.