I am a US citizen currently studying in Rome, Italy. I have a D type Schengen visa which expires in mid May, but I wanted to know if it would be possible to stay in Italy until mid July. When my student visa expires, am I by default under the regular 90 day tourist visa, or would I have to leave the Schengen area and re-enter to be granted the tourist visa? Or is there something else I am not aware of? I do not accidentally want to do something illegal. Thanks for your help.
I doubt after your visa expiration you'll be covered by the 90 days free visa waiver.
most likely you have to leave or ask for a visa extension.
I'd say that too butalas
best bet would probably be to contact the US embassy
After expiration of your D-type visa, there is in most cases no possibility to obtain a C-type visa / waivor. You'd have to leave the Schengen area, and you can only return after having spent 90 days outside.
If you want to stay on I strongly suggest you ask for an extension of your D-type visa. It is the best way.
You can also apply for a new D-type visa from another country while you are still in Italy, this would also fix your problem.
But in your case the D-type visa is your only chance. Bentivogli summed it up nicely.