I personally think spell sharing shouldn't be allowed. Maybe I'm biased, being a professor and all but besides exp, I'd say spells are the main incentive for people to attend classes. "But Veryna," you may be thinking, "that's why you can only tell a few friends! Surely it can't affect that much?" Well, let's say you tell two people. No big deal, right? But, imagine those two people go tell two people each. Now we're at seven people. These new four people all go tell two more people. Now 15 people know, and so on and so forth. Plus, I can't speak for every professor, but I personally would be kinda bothered if half the students on the server stopped showing up to my classes cause they already learned a bunch of transfiguration spells from friends. Plus, the amount of first years running around with Y5 or higher spells is ridiculous.
I also think spells should be restricted by year, ie Y1 can only cast Y1 spells, Y2 can only cast Y2 and below, Y3 can cast Y3-Y1, etc. I understand a lot of people like to work ahead and practise higher spells, so to accommodate for this the restriction could be for one or two years above, ie Y1 can cast Y2/Y3 and below, Y4/Y5 and below can be casted by Y3, etc.