Hmmm. I'm strongly inclined to interpret this as "their own chairs", mostly because of "quisque". As it's expressed now, it would say either "each stands before his own chair" or "each stands before the teacher's chair" -- but the latter doesn't sound quite right, does it? "Each" emphasizes the individual members. If we wanted them to stand before the teacher's chair, we'd more likely say "They all stand before the teacher's chair" (omnēs stant ante suam sellam -- in this case it's also grammatically clear that there's only one chair, or it'd be suās sellās).
As for whether "suam sellam" could theoretically refer to the teacher's chair if we ignored that context, though, I have no idea.
(EDIT: I looked at the sentence again, and I think even in that case it would have to be the students' chairs, because it's in a relative clause. I do know that use of sē/suum can be ambiguous in cases such as indirect speech, but I don't think it can be ambiguous in relative clauses...)
brookter wrote:Putō sē stultum esse...
If you're trying to say think you're stupid (which you're certainly not
), that should be "mē". Remember that sē is only used to refer to a third person.