Although not in a seminary, I am studying for a theology degree so I encounter most of what is being taught there.
I have found my beliefs seriously challenged at times, and in some cases had to re-evaluate, but I think that the important thing is that the majority (hopefullly) go into seminaries with a sincere desire to serve God ie they feel called, and they have a solid foundation of what they believe - so in seminary, it's only their interpretation of of those core beliefs which alters, NOT the beliefs themselves. (Hope that makes sense!)
That said, one of my friends went to train for the RC priesthood and quit seminary after 2 years, he had become an atheist, so it does happen sometimes!