Am I better off taking boxing lessons or a class?

I’m looking to start boxing, and I want to take it seriously—not just as a way to get in shape or something to do with my spare time. One-on-one sessions with a trainer cost a lot of money, while classes tend to go for only 10 or 15 bucks a pop. Those of you out there with boxing experience—if I want to get serious, and get good, will a big class be enough?