I'll give you a corporate point of view.

Many companies are required to have *compliance* programs, which include establishing clear policies for many situations, make the documents for such policies accessible for everyone (sometimes public even for people outside the company), and ministering presentations for all employees (required to sing an attendance list).

So, you can probably check one of these policies from a company whose reality you believe is representative of yours.

A common rule in corporate compliance is that you can accept gifts from bidders. It would often sound rude if you rejected them. But there is a limit to the likely value of a gift you can accept. Where I work it is 100 dollars. And the case is pretty much analogous, if we're talking a bidder, this person represents a company whose offer you'll need to evaluate. Though usually you need to pick a winner, rather than grading his proposal. 

If a gift is known or discovered to surpass the limit value, one option it to auction the item and donate the value for charity. Of course this is expected to be done without the gifter's knowledge.

Sometimes there are specific policies against accepting alcoholic drinks, but this is not so usual.

Up until here, I would said you pretty much should have clearance to accept this gift. And I would accept it if I was a teacher, given one last condition:

>How fair and objective is the grading system?

Could you be accused of favoring the student's grade because of his gift? If you are a math teacher who practices binary grading (i.e. each question is right or wrong, only the final result counts), no one would expect a test grade to change over a gift. If you are correcting essays which are related to subjective topics, the type where student would often argue that better grades are attributed if "guessing the teacher's opinion on the topic", then care should be taken. 

Even for the latter case, further trouble could be precluded by formally delivering a grading reference to the students, showing their graded papers/tests/homework such that any unfairness, if suspected, could be pointed out, clarified and corrected. Which sounds like good practice to me.

In the end, don't put yourself in trouble, have decent practices and nothing wrong should happen for accepting a small gift.