I thought I had the knowledge to build a nuclear bomb when I was in high school. In retrospect, I'm not sure I was right, but I'm pretty sure I was close to right. From where I was, I certainly could have picked up the knowledge as an undergrad; it's no longer a hard problem with publicly-available information.
For better or worse, I did not have the requisite $2B-or-so to do so at the time. My allowance was $2/week. I don't think most terrorist groups have $2B either. Even if I did have the requisite $2B, I don't think I could have done so discreetly enough to not get caught; someone would have noticed a high school student buying up things like uranium or building appropriate facilities.
I'm more concerned about biological warfare. While I don't have the knowledge to engineer a super-virus, we're at the level where a few years intense study is all it takes to have that knowledge, and a few high school students do. The cost structure there is increasingly moving into the budget of high school students too. Things like DNA sequencing and synthesis have a Moore's Law-style curve, with falling prices.
(If this sounds unrealistic, I was in high school back in the days when nerds intensely learned physics; physics is much less interesting to nerds in 2022. Hot topics for nerds change over time, and we're out of the cold war and the space age. Today's high school nerds have moved on to today's hot topics like machine learning and microbiology)
They could probably make one (it would probably fizzle though, but that's still nasty, even if not full theoretical power), they just don't have the highly enriched uranium to do it.
