[steven]

"Would you say the nuance is something like in the second example, there's a bit more emphasis on the "sometimes"? It kind of feels that way to me, since the most natural place I think for たまに is at the beginning of the sentence (if I'm right about this, I'm jumping for joy that my ear hasn't devolved too much over the years!)."

I think that on the first example the emphasis would be on たまに, and on the second example the emphasis would be on school. The reason being for this is the use of the particle は. It's like you're changing what you're talking about from something else to school. It's kinda like "ya I work full time and have 3 kids... as for school, I sometimes go" or "I'll go to school sometimes, but I usually stay at home and play video games all day" (which is an unfortunate reality for some students!!). I think in both of those English examples you can see that the emphasis is taken away from "sometimes" and placed more on the idea of "school vs. something else".

At least that's how I'm thinking of it at the moment! I could be misleading everybody, though.

Something I've noticed though is that on a lot of the google results for these things there is a negative connotation for most if not all of these examples. I didn't get that same feeling when searching "sometimes I go to school" or "I sometimes go to school" on google.