I can't stop crying at school. I'm 15 years old, my mom passed away 3 months ago, she had an inoperable brain tumor. It was very difficult illness and she fought and endured as long as she could, but we know the outcome. Sometimes I think that the only reason she tried to go on are my little sister and me. I should never cried in front of mom, it was surely as painful for her as for me, but now all I'm left with is crying.
Please, accept my sincere condolences. Crying at school is just a reaction of your mind on everything that happened to you. At school, your friends sometimes mention their parents, and that can easily be the trigger for your crying. You seem like a smart kid, so you now how bad this could turn out to be, in psychological manner of speaking, so you have to be strong not just for you, but for your little sister also. Without mom, she'll turn to you as her female guardian so be prepared for that new and premature role in your life.
crying is part of the healing process