Izuku knew Class 1-A since childhood but fell out with most all of them because they bullied him, Momo Yaoyorozu, Kyoka Jiro and Shoka Todoroki because he and Momo were misdiagnosed as quirkless, Kyoka's quirk was seen as useless and Shoka's quirk was viewed as a villain quirk for being really powerful but was able to forgive the people he fell out with but kept the fact he was married to Momo, Kyoka and Shoka a secret.
One day, during their second year of U.A, Mina dared Ochaco to ask out Izuku out of a desire to play cupid but Ochaco refused without hesitation which shocked Mina.
The reasons she refused was she knew about Izuku's marriage with Momo, Kyoka and Shoka along with Shinso, Koda and Tokoyami. And because she was dating Izuku's sister, which was also a secret.
However Mina was unsatisfied with this turn of events and she and the other followed Izuku home to force him to date Ochaco but they saw Izuku with Momo, Kyoka and Shoka along with little kids as the girls all discovered Momo, Kyoka and Shoka were with the guy they loved and couldn't stand for it.
Over the next few days, Mina spread rumors about the Momo, Kyoka and Shoka, saying they all raped Izuku and were abusing him which reached the staff and turns out they all knew about their marriage as it also reached Izuku and he was not happy with the news as it ultimately proved what he always knew.
They'll never change.
