OK Emma
Many have suggested that you should dive in and give it a go of adding to the forum - so take my word for it, that you'll benefit more by jumping in with a comment now and then.
So lets move on eh ?
OK I still dont have enough information to give you any sensible advice. OK when you say that you've set up a contract, does that mean that they are paying you to be there, or are you shooting the deb association (scuse my ignorance but can you elaborate on what that is exactly ? I can assume, but prefer not to. My assumption is that its a separate company that produce the management for a debs event - if so, I've been in exactly this position in the past).
If I correctly understand the position, and assume that they're paying you, the question from them seems reasonable. So, your choice is:
A) to accept that they're paying you, and it was their intention or reason to get you to shoot so that they could always use it as a commercial tool to increase their profits - and if thats the case, you might be on the end of the attitude that if you dont agree to do it, another photographer will.
or
B) to accept that they're paying you, and that you will stick to your original agreement, and no one else can buy the images (which is a waste, because their would be and is a market for the print sales after/during the shoot)
or
C) to accept that they're paying you, and you offer to put the images up on your own website or have them available immediately on the night. Which might go against their intent and they might end up charging you for the ability to take the shots in the first place.
or
best of all/my choice
D) to accept that they're paying you, allow them to manage, resell, and print production themselves, and you take a percentage (50-50 or 60-40 - thats in their favour, they have the costs) of each recorded print sale. They get the main challenge of handling the print sales, and you add to your profits.
How does that sound ?