She's really annoying me and making me feel depressed. What can I do?

We were in the same homeroom last year, and now this year. We weren't friends at the start of last year, I sat with some other girls and she sat alone. The only reason I knew her was because she hung round me and my friends all the time at recess and lunch. She got so clingey we would trick her by playing hide and seek and all meeting up while she was looking for us (desperate times call for desperate measures i guess). Then I felt sorry for her and became her friend, but I found that extremely difficult because she was always dragging me down in cl because she was a bit slow, and always talked (usually incoherent babble). Now she's making me depressed, she's always like "uugh i hate school, i don't want to come to school tomorrow" usually i just reply "join the club and stop complaining", but now she keeps saying that if she lives past the age of 60 she's gonna commit suicide because life will be boring after then, and that she won't have anyone to live for