Okay here's an interesting example of the sort of topic you could investigate for class. It only took me 15 minutes to get this far (thanks to the internets) although if I actually read the papers it would take a bit longer....
It started with a simple question - if mitochondria and chloroplasts have an endosymbiotic origin has anybody proposed an endosymbiotic origin for any other cell organelles?
Wikipedia (like I said, never a bad place to start) immediately suggests one possibility, the peroxisome 'they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere', and provides a quick review of what a peroxisome is:
Peroxisomes are organelles from the microbody family and are present in almost all eukaryotic cells. They participate in the metabolism of fatty acids and many other metabolites. Peroxisomes harbor enzymes that rid the cell of toxic peroxides.
However their article on the endosymbiotic theory gives a reference to a 2006 paper (Origin and evolution of the peroxisomal proteome) that states that
'Altogether our results indicate that the peroxisome does not have an endosymbiotic origin and that its proteins were recruited from pools existing within the primitive eukaryote.'
Time to leave Wikipedia. Is this the state of the science or have there been any recent updates? A Web of Science search for who cited the 2006 paper throws up a 2010 paper, The origin of peroxisomes: The possibility of an actinobacterial symbiosis in the journal Gene just this week, that reaches a very different conclusion:
We provide several lines of evidence supporting an actinobacteria symbiotic origin for the peroxisome.