Change of status from TD to F-1

If someone is on TD status, does the person need to leave the country to change to F-1 status or can the application for change of status be done by mail without leaving the country?

It can be done either way, although it's probably quicker to leave and re-enter and given that F-1 is pretty straightforward it might be easier to do that. Plus it may be cheaper if you live near the border (either border, I doubt F-1 would be a problem at the Mexican border).

Otherwise you can file I-539 and do it that way.