Izuku was framed by The League of Villains and nobody in UA believe in him, not even his own kids, Eri and Kota. Because of that, he lost 3 years of his life. Izuku almost died in Tartarus because his former class assaulted him and the teachers didn't do anything. Worse, The League Of Villains give him also a visit in his cell. Hisashi came and released the seal on Izuku powers, The Celestial Dragon. 3 years later, he was free after Tomura revealed that they framed Izuku and nobody believed him.