Breakfreefromplastic.org has some useful bits and pieces on plastic use, which could be helpful for market failure topics. There is a brand audit report which is handy but also a good source of discussion on methodology, as the "10 Worst Polluters" is based on physical waste found - I'm sure your classes might have some ideas on whether that is a good measure. Rankings like this are useful for 'higher or lower' activities, 'put these in order' or just 'how many of the top can you name?'.
Some resources on plastic pollution