Is there any basis for "bad birthday on the fifteenth day of junior high school"?

There is no basis at all. It's all superstition.

In the past, some days in China were taboo. First, the first day of the first month, fifteen; One is the fifth day of May; One is July 15. In the first month, it was a busy day for China New Year. If a baby is born in the first month, it is considered to be the baby's sexual poison, which will make parents uneasy. During the Chinese New Year, they are easy to relax and be happy. They will be very nervous and busy all their lives, and even have to worry, be afraid, suffer and be implicated. Therefore, children born at this time are taboo, saying that they will hinder their parents when they grow up. There is a saying in Henan: "Men are afraid of the first day, and women are afraid of the fifteenth." The first day is New Year's Day, and the boy born on New Year's Day is fatal. You can't beat and scold a child during the Spring Festival, so that the child's parents will not dare to discipline him in the future, which will hinder his parents. On the fifteenth day of the first month, it is the Lantern Festival, and it is time to watch lanterns. When girls are born at this time, adults can't see lanterns and enjoy little things. I am also afraid that girls born at this time will be playful and become "crazy girls" when they grow up, hurting their parents.