Stress
As a mental health professional, you'll help others deal with their own stresses. These are going to mostly be school-related, from learning and comprehension assistance to peer mediation to helping a student understand why it's not okay to write "Johnny Finklestein has sweaty pits" in the boys' bathroom (even if he totally does).
Your own stresses will also be school-related: your hours are long, your working environment can be crowded and decrepit, and while most of your "clients" are probably great people, there is that portion of people who are so deeply awful that it will make your night job as a UFC fighter the least violent and angry part of the day.
Where you work in the educational system probably won't change your stress level; the stresses aren't greater or lesser, they're just different. In elementary school, you'll have to work with children who absolutely do not understand why it's bad to stick their lunch fork in their friend's eye, while in middle school they'll totally know why it's bad but they'll do it anyway. By the time they reach high school, you'll be lucky if it's just a fork.
On the bright side, as part of your profession, we expect you'll know plenty of qualified mental health professionals who can help you deal with your stresses. And with all the students you may have to scream and cry about, remember to say a small thanks for doctor-patient confidentiality.