I'm not a Ruger fanboi...and there are certainly some QC issues with Ruger...but honestly I think I'd look for a different instructor on two counts:
1) If I were instructing then you'd be signing a waiver that if your gun failed during the class and you didn't have another to continue to finish the class...it would be your tough luck.
2) Given (1) above, I wouldn't see much need to say, "no, your 'brand X' isn't good enough." Now, I might make recommendations based on experience if you were wise enough to ask...but to simply say that a particular brand wasn't good enough for my class is kind of lame unless the concern is real safety issues (depending on the nature of the class there are some individual gun models especially among the cheaper imports that I might well disallow).
The bottom line is that any instructor has a right to set his policies and if this guy isn't comfortable with Rugers that's his right. The other side of that coin is that potential students have every right to decide where they'll spend their hard earned dollars and I'd be pretty skeptical of spending mine with an instructor that blanket-banned Rugers.
As I said, it's fairly obvious that there are some pretty serious QC issues with Ruger right now; at least, it seems obvious to me having purchased a recently manufactured Ruger pistol and having had to spend a considerable bit of time doing finish work that Ruger should have done just to make the gun reliable with common defensive ammo. That said, there are tens of thousands of Ruger pistols out there that are reliable and proven...eliminating them smacks of elitism or laziness, IMHO.
John