There is nothing you have to enter in Turbo Tax or your tax return. Just enter the W2 exactly as shown. It should not have any Social Security on it in box 4.
You don't need to address this on your tax return. Your employer will deduct the PERS contribution from the gross wages in box 1 and you can't deduct it again.
It is not news to TurboTax that many teachers do not pay in to Social Security. Many teachers pay into teachers' retirement pension funds of various kinds instead of paying in to Social Security. All you have to do is enter your wife's W-2 exactly as it appears--with nothing in box 4. You just leave that blank when you enter the data from the W-2. And nothing about what she paid (or did not pay) into Social Security goes on the actual Form 1040 anyhow.
Teachers and Social Security
There are 15 states (AK, CA, CO, CT, IL, LA, MA, ME, MO, NV, OH, TX) in which teachers do not pay in to Social Security, so you will not see Social Security withheld on the teacher’s W-2. Instead, teachers in those states pay in to a Teacher’s Retirement System, which will show up elsewhere on the W-2 often in box 12 or box 14.
There are three states which have varying degrees of participation in SS for teachers, depending on school districts—GA, KY, and RI.