As you say, it may or may not violate university policy, but I do think it may cross some boundaries either way. 

Let me put it this way - if a fellow lecturer came to me and asked me whether it's appropriate to organize such an event I would advise against it: 

 1. It excludes some students (in particular, those who are probably less sociable than others, those who don't drink or have dietary restrictions, or just those who don't like the lecturer). 
 2. It offers students a plausible argument against the lecturer in case they get a bad grade: it makes it appear as if they are playing favorites. 
 3. There is a power imbalance: grades aren't out yet, and students may feel like their grades depend on how nice they are to the lecturer (they may not be wrong!).

In general, socializing with students when a course is ongoing and their grades are still not set is not something I would *not* recommend. Students like you who are uncomfortable and worry that their grades may suffer are exactly the reason why.