All J-1 visaholders are exempt from Social Security and Medicare taxes for at least the first year and first semester of the second year in the foreign teacher exchange program.
Download p. 36 of IRS Publication 515 verifying that J-1 visaholders are exempt from Social Security and Medicare taxes.  https://www.irs.gov/pub/irs-pdf/p515.pdf

Click here for IRS Tax-Treaty Publication 901:
TAX Treaties

IRS TAX EXEMPTION / REFUND FORMS:        8233      8833     8843      8316

TAX instructions for UK Teachers:  If you see a pop-up window asking for a password, simply click on CANCEL.Additional tax-exemption information is offered at the CHF fall orientation sessions for new J-1 exchange teachers.  CHF does not guarantee tax advice, only directs educators toward publications and forms that may be helpful.