I'd say of any period of my life high school (14-18) were the low point. Here's how those four years felt for me.
Wake up early and groggy, go to school, go to work, get home and study, go to sleep late. Repeat.
I had little free time, and was constantly saving money for car and plans for university. I didn't have much of a social life, and frankly puberty was not kind to me. I frequently felt isolated from my peers and dealt a lot with rejection.
Personally university was so much better for me. I felt more engaged intellectually with my professors and peers, and I had more free time for a social life as I worked less (thank you scholarships) than I did in high school. I also got on the right side of puberty, lost weight, lost acne, and had corrective surgery for my eyes.
Moving into the "real world" I was told horror stories and how high school was "the best years of my life". Not true at all in my case. I had freedom, I traveled, run my own business, had success in romance leading to marriage, and have made the best friends I ever have had. I wish I could tell 17 year old me, "One day you're going to be happy as a rule rather than on an occasion".