What a Husband Should Be by Sheila is one fan fiction story where the judge helped with the adoption and forged/created a birth certificate.
Egg allergy works for the viruses influenza and yellow fever because those two vaccines are grown in eggs. Tetanus vaccine a weakened version of the bacterial toxin, and not grown in eggs.
The only contraindications to tetanus toxin vaccinations would be severe allergic reactions to a previous injection.
CDC Contraindications and Precautions to Commonly Used Vaccines in Adults 2016 The doctor may not have had to document the allergy to get Clark out of the injection. Simply saying he was "medically excused" may have been enough for the school system in that era. Of course in a small town the school doctor may have been the only doctor in town. He may have just have had to verify the immunizations were up to date on official forms, and not document the details.
Addendum: I'm sure you're mother is right about the regulations and reasons for deferring vaccinations then. I wasn't sure what they were, but all of those things make sense. Sorry I didn't see your post before replying.
