You're the driving instructor arent you?
In that case, I wouldnt bother as as you mentioned, your pupil will definitely be kerbing them!
Wheels were definitely at the top of my mod list but its a case of finding a decent set for a decent price (ie 2nd hand).
As I mentioned above, for a decent good make alloys, brand new- you're talking in the region of £800+ for 17" plus tyres.
2nd hand, you're looking £400-500 for a decent set with half decent tyres left, and most of the time, you will have to spend in the region of £200-300 on refurbishing them to make them as good as new.