boy whose girlfriend dies and has psychosis

this book mainly focuses on this boy thinking he’s time traveling but he is actually going through psychosis if i remember correctly. his girlfriend committed suicide before the events of the book but he thinks she got lost in time and he keeps trying to find her. the book takes place at a school for children with mental illnesses but he is convinced they all have superpowers for example a classmate with pyromania he believes has fire powers, another classmate with anger issues he thinks has telekinesis. i remember the cover design was white and looked like it was wrapped in string. also he thinks his girlfriend is trapped in salem witch trial time and her “power” is invisibility (depression).