My biology teacher told us about this BLB from a few years ago

I was always under the impression that most adopted kids were told at a reasonably young age that they were adopted to avoid emotional problems in adolescence. I don't see why parents would withhold that information from the kid, who I assume is in high school and thus an adolescent...?

While learning blood types in bio, my biology teacher told us that we had to use fake blood because 20 years earlier, students used their own blood and one girl got the recessive blood type O (only possible if both parents have O blood). The next assignment was for the girl to find out her parents' blood type. She went to her mom and as expected she had O blood. She the went to her father and to their surprise he had type A blood. The mom had cheated 15 years earlier. As a result the parents divorced, then sued the school, and the super intendant demanded that only fake blood be used from then on.