Answer 1: There are a few possible halachic/legal models for the nature of your arrangement with the yeshiva, which would impact elements like those you ask about.
It is possible that you were an agent (shaliach), buying food on the yeshiva’s behalf. If so, they must reimburse you in full for what you bought as their agent, and the food is theirs. Then you would have to determine whether they allow you to eat their food even after the time during which you and all other invitees clearly had permission to eat as much as desired (i.e., during the oneg). One may assume the yeshiva does not care what happens with small amounts left over in open packages. Regarding the rest, it likely depends on various factors, including the management style of the yeshiva and the extent to which it is worthwhile for them to store the food for a future event, refreshments for workers, etc. Even in cases in which one is confident that the owner of an object would be happy with a friend taking his object, there is an unresolved machloket regarding whether it is permitted[2] or forbidden[3] to do so without the owner’s knowledge.[4]
Another possibility is that you are viewed as one who bought the food for himself, with a promise of compensation. If that is the case, the food is yours, and you may do whatever you want with it. However, this raises a different question: How much compensation may you ask from the yeshiva? If you do not take the food for yourself, then they probably have to compensate you for all that you bought, and they cannot require you to use, on your own account, that which was not eaten at the oneg. However, leftovers that you do want to eat turn out to be food that you did not use for the group, and it does not seem that you should be entitled to ask for compensation for them. On the other hand, the value to you of the leftovers (certainly the open packages, but likely even some closed packages) may be less than the amount you paid for them in the store. Therefore, you would not have to reduce the full face value from your request of a refund, although likely close to it.
The first possibility, that you acted as an agent, seems more likely, but you likely know better than we do.
We encourage stringency on matters of monetary ethics. The wisest stringency is often to raise the issue with the relevant authorities, together with a smile, hakarat hatov, and willingness to either forego taking the leftovers or to pay for them. In cases of good relations and only a few shekels at stake, it is normal for people to be generous. Asking for permission or for guidance as to how they want you to handle the situation not only removes the question of impropriety but likely will get you a good arrangement in the present and builds trust and removes doubt for the future.

Answer 2: While the school might allow it, it is not so nice to ask the school to pay money for something that its students did not benefit from at all. On the other hand, you acted with good intentions, and there is no reason for you to lose money trying to do the nicest thing for your students and being honest with the school.
Sometimes “practical advice” augments halachic advice importantly. We suggest that you make the expensive salad even if you now think that you have a better idea. We are sure it will be sufficient, and will be worth doing so as to avoid the moral dilemma. While we generally are in favor of asking the appropriate authorities what they want you to do, in this situation asking them your question may put them in an uncomfortable situation.