Students should be encouraged to develop a thesis that is specific. If the topic they've chosen is too broad, they should consider what aspect of it is most important, and work to make a specific statement about it. For example, if students want to focus on fair treatment of people, encourage them to think of specific groups who may experience unfair treatment. Then encourage them to think of a specific reason we should change this treatment, or a suggestion for treating these people better. The more specific the thesis, the more focused the writing will be.
A thesis, by definition, must be arguable. If students are simply stating a fact, they have not developed a sufficient thesis. Once they think they've arrived at a thesis, ask them to share their ideas with a class partner, and see if the partner can argue against them. If they cannot, their thesis might be a fact rather than an argument. If they are stuck on a fact, ask them to consider why they think this fact is true. The reason they come up with will likely be closer to a thesis than the mere fact.
While it may not be possible to prove a thesis true, students should certainly be able to find evidence to back up their theses. Therefore, they should choose theses that allow them to find evidence to support them. If they think of something too abstract, or that is merely an opinion, they will become stuck as they try to support the idea. Encourage them toward current events, history, or even an issue in the school for which they can find data.
Finally, the thesis statement should make the reader want to continue reading. While sixth-graders are only beginning to develop their writing voices, encourage them to use lively language and attempt to draw their readers in. The argument they are making should elicit controversy in their readers, and should make some readers disagree, thereby causing their essay to be more interesting as a whole.