I would strongly object to your teacher about this. 2 Litres in NO WAY should house a very large messy fish like a Goldfish (or indeed any fish). This clearly displays blatant ignorance on proper fish care on your teachers part. THEY need educating!
If you must do a project like this, use Ghost Shrimp, NOT Goldfish. I imagine your teacher is thinking somewhere along the lines of the Walstad Method:
http://www.theaquariumwiki.com/Walstad_method
If you want a Goldfish to thrive you need 75 litres plus, over filtered and has completed the nitrogen cycle for 2-6 weeks before the fish is added.