As you have mentioned there is no photography clause as a stipulation to your employment at the college the images, therefore, belong to you. Lock, stock and barrel.
The question is not about ownership but permission to use. I would assume they came to use them through a verbal request. That's usually how it starts; "hey, these are good. Do you mind if we use them for such and such?" Which you probably gave consent without thought. Why not, it's rather flattering.
By prior arrangement, they already have the right to use the images. You gave it to them and they can point to past publications as proof. Chalk that up to experience. At this point you have to decide on how you want to proceed from this point forward with any additional usage of those images and any new ones you produce. This can be a tricky spot for you because prior consent makes them assume you'll continue to cooperate. If you switch and expect compensation (which is within your right) they may take offense, citing past donations.
At this point it becomes a matter of what you are willing to live with. If you don't need to be compensated and like the idea of your images gracing their publications then keep things as is. No harm, no foul. If you want to earn a little side cash for your efforts then sit down and renegotiate your agreement and write up a new licensing proposal. It doesn't need to be fancy, just something both you and the college can agree upon.
If none of that matters and your only concern is about ownership, rest assured, you own the images. Prior use in their publications does not transfer ownership to the college. However, without a proper licensing agreement they can use those images as they like.
Hope this answers some questions.