On what line of the medical expense form should private tuition for special education be posted?

Education expenses are not an itemized deduction.

Education expenses for grades K-12 are not entered on a federal tax return.

In TurboTax, Enter at "Any other medical Expenses".  

There is no special place on the actual IRS forms. On Schedule A, it is just lumped in with all other medical expenses on line 1.

Special Education for medical reasons would be included on Schedule A line 1 along with the rest of your medical expenses.

As you walk through the medical expense screens in TurboTax you will get to a screen that lets you enter medical expenses that are not listed.  You can enter the description there and the amount.  This will flow to the worksheet, that will flow to Schedule A line 1.