Read the article excerpt. You can get a blood test to prove you have immunity.
So if you got the vaccination when you were 5, you’re okay with the school, even if there’s very little immunity left from it (why we get booster shots from other diseases - like tetanus, MMR, etc.) However, if you got the actual disease at age 5, and it’s in your medical records, you still have to go get a blood test to show that you have enough of the immunity left from it? Niiice.