Ha. We were required to use Poseidon for UML, but it was such a pain and crashed so often that we ended up making all our UML Diagrams in Dia (which is a nice enough and lightweight program, if slightly clunky; you might want to check it out). Yet instead of being pleased or even just ambivalent about the entire enterprise, our teacher got mad that we used a tool other than the prescribed one. I never really understood why people would get pissy about what tools you used instead of focusing on the actual output.
Maybe your teacher's goal was to encourage people in the class to learn Poseidon so he could hire them on a private project. Or maybe he/she was looking for a genius who would be able to figure out how to code Poseidon in such a way that it would not crash.
There are lots of other reasons why a teacher might set specific rules for an exercise such as this, too. Unfortunately you chose to ignore the instructions -- and you made the mistake of second-guessing him/her -- assuming that the teacher's goal was the end result and not the process of getting there.
