It does sound like she could be having slight depression. Being fresh out of high school, I'll tell you a lot of what she's going through sounds very similar to how I was. I was depressed, but very few ever suspected it because I always seemed to smile around others so they wouldn't worry about me. I was suicidal for a long period of time and had my own brush with death at that point.
If she is quiet and has trouble with her feelings, seek her out and ask her how she's doing in school, try to be social with her and let her let things out. Sometimes, all teenagers ever need is someone they can trust to talk to.
If that doesn't work, give her space, but not to much. Keep letting her know you are there for her and that she is not alone. As the mother, a teenage girl falls back to her quite often, so your outward acceptance towards how she is doing will really help. I know, because it was my mom and grandmother that really helped me with keeping a sane head through the hard years I went through with my depression.
She could be depressed. Usually when people start searching out depression frequently, there's some truth to it, some feeling of not being alone. I would probably take her to her psychiatrist and ask for a diagnosis. ADHD shouldn't cause suicidal thoughts alone, but it is co-morbid with other psychiatric illnesses at times.