Being that the agreed deal was volunteer hours for lab usage, the agreement was void once they took away the usage. The posters were just a by-product of that arrangement and by no means their property. Displaying the posters was merely a mutual benefit to the both of you while trading time and space. It's not like working in a design studio where you're being paid to create something and the studio owns the rights to the work created in that studio. I'd say you have every right to your posters. Best of luck, hopefully things go smoothly for you.