Translation:That class was boring.
Probably because "I" is not the topic; "class" is, as marked by the は particle. If you wanted to make yourself the topic (without having to explicitly state it with 私は) you would have to make "class" not the topic by changing its particle (perhaps to を), as you can't (in most situations) have two は particles in the same sentence. It would go something like this (I think):
I hope that helps!