The empty form on page 23 has the kanji
離婚 "Rikon" which means divorce. Nagisa's parents appear to have divorced, and it's almost certainly related to Tsutsuji's manipulative toying with her "friends".
Please don't make an additional reason for Nagisa's trauma be sexual abuse... but considering how many close calls this manga has about sexual assault, to say nothing of the protagonist's trauma... I'm absolutely not gonna be surprised if it is.