We list that under the duties of our treasurer, since it is financial. You could also look into a CPA if no one feels comfortable doing them themself. You may even have a parent that is an accountant & would do it for free/low cost. But, ultimately, where it involves money/the accounting of money, the treasurer should be responsible for seeing that it is done-whether they actually do it or have someone else do it.
Our PTO has been remiss in filing its forms. Although we are a 5013c, that doesn't excuse it. In digging in to this, it seems that at least part of the reason is that no one has the official duty to do this - the by-laws list the duties of each officer, and none of them have the duty to file tax-related forms!
I'd be interested in hearing how other groups handle it. My first reaction is that it's the treasurer, but before we ammend the by-laws, I wanted to hear some others' input.
