You told them they could use the images. You got in free. If they chose not to use them after the fact does not change the agreement. Either way, from what you posted, they are free to use them or not. It is up to them how they use them in the yearbook. Maybe it doesnt match up with what they are going for this year. If they have for sure said they will not be using any of your images, then you have a right to ask them to delete them from their archives and not use them at all. But in the end, it doesn’t matter if they dont use them. You got good images for your portfolio and that’s what you were after, right? Well done!

Keep shooting and hope things change in the future. Personal advice, it’s not worth a lawsuit if they decide to use them or not. You will be blackballed in the future as someone that is difficult to deal with. And you don’t want future paying clients to hear that.