Yeah that situation the OP described sucks and was pretty harsh for the school to do but I've had higher weightings for random tasks too (I'm sure almost all of us have). For example, my Ancient Trial was only worth 20% and some random in-class essay we were given the question for was worth the same. In Extension 1 English our speech on related texts was worth 40% with the trials at 30%. You can't control these things!