A friend in high school (not close).
Couple of students.
A romantic interest.
It’s always disheartening.
A 19-year-old nephew died suddenly from a virus. He felt sick and passed out. His mom called the ambulance and he died on the way to the hospital.
A 16-year-old friend died in her sleep of an aneurysm.
Growing up my neighbors had a niece/granddaughter/cousin that dropped dead at a ball game; some kind of undiagnosed heart problem.
I also knew a 16 year old boy who died at home after basketball practice. Another undiagnosed heart condition.
A guy a few years ahead of me in high school dropped dead on the football field. It was not a particularly hot day or a particularly hard practice, from what I hear- he just dropped dead.
It was '94 in England - I was wrong about her age though, she was 26. The tragic irony was that though her peers like me were mainly dissolute smokers and drinkers, she lived a super-healthy lifestyle. She and her boyfriend went to bed that night with nothing wrong with her, and he awoke next to her body. I can’t imagine how terrible that must have been for him. 
I have known four people, all in their teens or twenties, who died of brain aneurisms. In all three cases the aneurism was never diagnosed, it just burst one night (as it happens all three died in their sleep). I know it’s all a coincidence, but it’s getting to the level of a Damned Freaky Coincidence.
Two kids at my daughter’s school died during basketball games. Both had a sudden cardiac arrest and could not be revived. I’ve always wondered if they would have survived had the school owned a defibrillator. Both kids were 13 at the time of their deaths, one year apart in the same gymnasium. I also knew a woman who lost her 3-year-old daughter when a strep infection invaded her heart. One night, she put her mildly febrile daughter to bed, and she just never woke up. It was very sad. 
A high school classmate of mine died of a cancerous brain tumor his senior year.
A guy who graduated with me drowned a year or so later and another guy who was a year younger than me OD’ed his senior year, but those probably fall under “accident”.
A girl who was a year or two ahead of me in high school died recently (she would have been in her early 30s), but the cause wasn’t listed in the obituary. She was diabetic, but I don’t know if her death was diabetes-related.
A college classmate of mine went on to join the CIA and was killed in Iraq. She was in her late 20s. I didn’t really know her, but we were in the same year and I used to get her mail by mistake because my mailbox number was the same as her dorm room number and her family kept addressing things to her dorm. I felt really sad when I heard she’d died because her family sent her a lot of mail, certainly a lot more than mine ever did, which makes me think they were an unusually close family.
When I was in 9th grade a girl collapsed in the pool during swim practice and died. It wasn’t drowning, but something to do with her heart.
A friend of mine dropped dead on his way to school, from an unknown heart defect. I was a freshman in college and he was a senior in high school.
Also in college, a girl I knew was brutally murdered.
And in the late 60s, I knew people who had died from drug overdoses.
Are we including war? I knew guys who died in Viet Nam.
A cousin who died of SIDS, a neighborhood boy was murdered by a sexual predator when we were both in junior high, a friend of the family was killed by a hit and run driver when she was 16 - technically an accident, but the “run” part makes it a crime in my book, an acquaintance in high school died of cancer, another was stabbed and killed in a street fight a year after graduation, the roommate of a girl I used to hook up with died from an inoperable brain tumor before he was 25, a friend in FDNY was killed on 9/11 when he was 28, the fiance of another friend was also killed in WTC1 she was 24.
If I were to add accident and/or suicide the list would easily double.