What should be done if a student had a positive tst in the past?

TST. Are you referring to a tuberculin skin test? Has the student been vaccinated? Vaccinations will cause false positives. Exposure to TB can cause false +. After a positive tst, the student should have a chest x-ray. If cxr is negative, chest x-rays should be used in lieu of skin tests every 2 years.
See a doctor. Anyone with a positive test for TB should see a doctor or public health clinic and get a plan for either treatment or surveillance. Do not let this wait.